
Once of the best things she can do is eat. you're body needs nutrients to grow and if you don't eat enough, you won't have the fuel needed to grow boobs. That said, some part of it is genetic however breast size doesn't impact her ability to breastfeed in the future. It's possible to induce lactation in just about anyone with enough effort, even cis men, so that shouldn't be too big an issue
also they won't exactly make boobs bigger, but a push up bra would be a good idea to try. it could even be a fun thing like you take her bra shopping or she can buy them and do a try on or something like that (keeping in mind this may be a sensitive topic, so her comfort comes first) so it's actively a good experience