If the rims were shodded  with the same tires, I say the difference is minimal. for 22inch or 451 , the tire choice is limited to thinner tires only, the widest is 1.3 inch. the wider the tires, the more resistance, meaning it needs more effort to pedal, especially uphill. But on the other side of the coin, smaller diameter makes pedaling easier. So, there are pros and cons. So this means  it is actually easier to pedal a 20 inch foldie than a 22 inch unit. Why don't you change the existing tires with smaller width tires, like the Kenda Kontender. It will ease the pedaling but it will be bumpy as narrower tires needs more air pressure and will be hard on your bump.

Changing to bigger rims makes pedaling harder, Adding another chainring will definitely help. This will add more lower gears, thus making pedaling much easier BUT it will be slower. Riding uphill needs practice and you need to build your stamina over time. You did not mention what kind of bike nor what size the front chain ring is. It will be easier to suggest if you did. Check the number of teeths your chainring have, if it has 53 or 52, you can change to 48 teeth unit and this might help.

Summary 

smaller diameter rims easier to pedal than bigger
narrower tire width easier to pedal but harsh ride because high pressure
the more gears you have, gets more choice on ratios, more lower gears means each gear drop, gets easier and easier to pedal but slower speed.

Many are quite wrong about smaller wheeled bikes. Every size is compensated by gearings in limited situations. A 20 inch folding bike can be ridden  18/20kmh cruising easily( for us I mean). A Brompton is a 16 inch bike and so are many more. It all comes to gearings. As I mentioned earlier,the smaller the wheel,the easier to pedal but not necessary faster, just pedaling effort s lighter. I think it is not worthy to add another RM400 to add another chainring but consolation though is the better added components can be transferred to the new bike if you changed bike later. I doubt she can be pedaling like hamster when going uphill,that means she is strong. Furthermore, one can go higher gear if pedaling is too light.

Bigger wheels does not equal to better efficiency. You need energy to turn them. Every inch the tires grips the terrain, there will be friction. That's why road bikes uses the narrowest tires for less resistance and the bigger diameter for  more distance per turn of the crank. But this needs energy just like a smaller wheels. If you see mountain bikes, their cranks goes to 44 teeth only as opposed to road bike's 53 teeth ,because mountain bike tires are wide and resistance is high. Off course in a race, speed is required but you also need to be able to finish the race. To tackle over rough terrains, you need many low gears and that's why mtb have the widest range.

15km/h is not that hard actually. I think just change to 1.5 inch tires or 1.3 tires like Kojak and if that still dn't help, change to 48 teeth cranks. That will make pedaling much easier but you will have to be slower to accompany her.

leg not strong, can slowly pedal up the hill.

if u want get new bike, get one with double crank should be ok oredi. 

change to smaller width tyre, can't really feel the difference. 

if your current bike can remove the crank or just ring, can change to 48T, 
can get very cheap frm taobao. 

http://gd2.alicdn.com/imgextra/i2/20375632...!2037563243.jpg 

largest cog is how many teeth?

can also change to 34t freewheel, also very cheap in taobao...

http://gd1.alicdn.com/imgextra/i1/11015918...!1101591844.jpg 

can u take some pics so can show us here   

what brand is the bike?

pic of the rear rd with freewheel.

nid to see the rd can accomodate 32t ornot. 

24t so small? can up to 28 or 32t if the rd can handle, 

front is 52T? if no change mebe need to add chain? need to check with wong sifu   

if front drop 4t back add 4t chain no nid change gua...

pic of the crank as well. 
shud be a tapered square...?
http://www.parktool.com/uploads/images/blo...lp/crank108.jpg 
crankset is very cheap oni frm china, no nid to change the bearings if is tapered square. 

u can bring to bike shop to pasang mar, won't cost a lot la.

oh ok then rmber to double check when u are back home,

u need at least 2 special tools, one to remove the freewheel and one to force out the crank.

the cost of freewheel and crank is cheap from taobao only 
freewheel less than rmb 30, not rm ya.
cheap crank rmb 50 or even less.
shipping, say 2 kilo rm 23 only by air.

if the rd is shimano tourney tx35 it can take 34T.

if tapered square bb, is all same size, no nid change bb,

7 speed usually have 14/28 cogs freewheel. Can't be a 24. Shimano Tourney can handle 32 teeth at the rear. To remove the crank you have to get a crank puller. The crank (single chainring) can get a Prowheel from Kimbell Jaya for only 70 bucks. But I don't know if he have a 48 cog ones. I know Dahon Boardwalk uses the 48 teeth as original ones.  I can assure, they are square tapered BB. All cone bearing BB are square tapered. You can change your freewheel cogs to the Shimano Megarange type.

One thing for sure is, if you really invest in a lighter bike, both of you will be really fast as you are already used to this heavier bike. Just like when I gave up folding bikes and went mountain biking and then I rode my velo to a ride, it was effortless.

7 speed usually have 14/28 cogs freewheel. Can't be a 24. Shimano Tourney can handle 32 teeth at the rear. To remove the crank you have to get a crank puller. The crank (single chainring) can get a Prowheel from Kimbell Jaya for only 70 bucks. But I don't know if he have a 48 cog ones. I know Dahon Boardwalk uses the 48 teeth as original ones.  I can assure, they are square tapered BB. All cone bearing BB are square tapered. You can change your freewheel cogs to the Shimano Megarange type.

One thing for sure is, if you really invest in a lighter bike, both of you will be really fast as you are already used to this heavier bike. Just like when I gave up folding bikes and went mountain biking and then I rode my velo to a ride, it was effortless.

that is a square taper type of bb.if u want to used hollowtech bb, u need to buy the whole crankset

Most common and easiest to find sealed bearing bottom bracket. Square tapered BB. 

maybe this would help the long explaination

https://en.m.wikipedia.org/wiki/Bottom_bracket

Any1 can comment on Cronus Solder 2.0 fold-able MTB?? Thinking to get 1 at the price of RM1960++

Izzit a good deal??

Thx..

Hi, thanks for the reply. Yup, my budget is a bit tight, im a student actually but i have been wanting to buy one so for a start mayb im gonna buy something affordable. I did survey for folding bikes online within my budget n found a few in lazada wirhin the range rm239 and above, with the specs being single speed or 6/7 speed. I also found one model, 21 speed mongoose bike at rm499. So would like to ask that in my case, do i really need to buy one with more speed since im just going to use it for leisure (for now)?

Thanks PaulKong brother for sharing the knowledge.

I most probably will go for Dahon. But up to this point i still can't decide to get Vigor or MU D8 or Speed D8.  Price around the same.

Was shortlisted Vigor & MU but saw many rider here own Speed and give very good comments on Speed.

So ................................. Pening     
hhahahahhaa

GUys,

Went to tommy's shop today. very sorry trouble him so long but never buy anything from him yet. was plan to get Vigor but saw IOS9 is really temping.
Any bro review here ? thanks