Buy the loose diamond and use a temporary ring holder to hold it temporarily. With this, you still can surprise her without risking choosing the wrong design and ring size.

There is a few options you can go about this:

1) Buy it yourself and pray that she likes it.
2) Bring her along and she can select what she really likes.
3) Check out her pintrest account (if she has any) or check out her "like" photos. Hopefully, you can locate a few.
4) Berpakat with one of her good friend, someone who will be discreet enough and find out what design she may like.

I personally went with no. 2, I went with my boyfriend to get mine. But that is my preference cos I'm quite picky
I've also done No. 4 for a friend. This is not going to be easy cos her friend would need to know how to start that conversation to get the gist of what she likes.

I would suggest you to bring her with you cos everyone has different taste, no doubt a solitaire is the norm but if she's as picky as me, I would go with No.2. You can always surprise her in the proposal itself since purchasing it doesn't mean engaged straight away

He has not proposed yet. LOL thus why I used "bf"
What we did was, he brought me and I selected. Then he kept it locked away and ensure I didn't have even one picture of the ring.
So that when he propose, whenever he proposes, I won't really remember how the ring looks like. Which works cos I can't actually remember how it looks like at the top of my head. I still don't know when the proposal is going to be so I'm sure I'll be surprised, even if it's not, it wouldn't change my answer

I have a friend who did pretty much like me. The guy took her too and kept it. 8 months later he proposed.
It was an ordinary day where he just told her "Let's have a date night". So she got dressed up and put on make up etc.
I was hiding behind. So when he popped the question, and she said yes, we came out congratulating them.
It was still a surprise to her cos although she knew a proposal was coming, she still didn't know WHEN.
So it worked

Actually I have quite a number of friends who did a surprise proposal rather than a surprise ring + proposal. It's quite common nowadays.
I don't think I actually have someone who didn't know about anything at all

Just a tip though: If you are planning a proposal, be it option 1-4, it's always best to still berpakat with her friends. As a lady, you would want to take nice photos of the ring after the proposal, a GOOD MANICURE is a MUST! Have her friends have a girls' day out the day before to get their nails done, trust me she will be happy. I did that for my friend mentioned and she was sooo grateful I did.