My experience isn't really doll-related, but I've dealt with my mother not liking some of my interests or hobbies. A part of how to deal with it might still depend on how old she is, and what part of the hobby her parents disagree with.... Is it the price tag the dolls come with? Is it because she's "growing up" and should be "too old to play with dolls"? If it's the price tag, and she has a job, she can point out that she's not asking for them to fund her hobby, and that she plans on paying for it herself. If she's young enough that her parents are telling her that she shouldn't be wasting her money on dolls, then as has been pointed out to me, when I start feeling guilty for spending large sums on dolls,
lots hobbies tend to get expensive in one way or another, if you're genuinely serious about them. If it's her age, she can point out that the hobby is world-wide, with members of
all ages, and the overwhelming majority are people who are full-grown adults, with very few who are children. These are collectors items, not toys, and there are online communities with users numbering in the thousands, dedicated to the hobby. She's likely to make friends all around the globe. The hobby is harmless, and it's an extremely creative pursuit. Seriously; doll people are some of the most ingenious people I've met. Mention all the artistic aspects of the hobby; from painting the dolls, to make clothing, accessories, and furniture for them.
If there seems to be no possible way to get her parents to like the dolls, then it may be that she needs to just accept that they're not going to like it. In my case, I actually had to keep some of my interests a secret.
Hopefully, that isn't the case for your friend.... I still have to keep some interests a secret, but as I'm no longer under her roof, it's not as hard as it was when I was in school.