why not try his gym routine without the mass gainer first? you say you dont want to be more than 80kg's, and your current weight is 80kg's. so why take a weight gainer?
and please elaborate on how your friend manages to lose weight from 78kg to 74kg while on a bulking phase?

dont matter how many days he train, bulking means to gain weight, not lose weight

any reason why your friend recommended you a weight gainer in the first place? why not take protein instead?

you probably dont understand what "bulking" is.

bulking

supplements are only one factor in bodybuilding. how much your bro and your friend are pushing it in the gym makes a difference. their daily diet makes a difference. genetics make a difference. how much sleep they get makes a difference. etc etc.

but YOU have to understand, mass gainers has more carbs than protein, and its ONLY use is to gain weight. whey protein contains protein, and protein is used to make muscle. read up and understand more. dont be the "monkey see monkey do" types.