Yeup, should be easy in fact. Those who are that big don't actually have higher hormones, instead they tend to be hypersensitive to hormones. So the herbal stuff tends to be far, far more effective on them.
Head over to the breastnexus forums and browse a bit, use the search tool. She should have an easy time growing several cups sizes. One already huge girl bustybride on there went up 7 inches / cup sizes with just a little pueraria mirifica I believe, ~H to ~N (plus a little band increase). That's about the simplest most haphazard method that others don't respond as well to. Again, not normal for most, but typical for huge chested girls. Find information on properly using pueraria mirifica and progesterone cream for example. Start low and work up to make it easy on her; no need to rush. A bit of *50% saponin* fenugreek or shatavari capsules is helpful too and more healthy. 2 scoops of collagen powder (instead of 1) would be good to help firmness since that's always an issue with large chests. More than the normal amount because she has more in her chest. But not 10 times more because collagen is also in the rest of her body. Such as joints and skin. Massage with any moisturizer or oil prevents stretch marks, and makes for good couple's fun. Also helps growth a tiny bit.
It's more a matter of how big does she want to go and will it be practical to be that large, since it shouldn't be difficult to grow. But otherwise I'm 200% in favor of having fun if you both enjoy it.
Stay away from actual hormone therapy. Even though she'd take off like a rocket, it also carries a much higher risk of breast cancer. That's the big theoretical scare with all the herbal stuff too, but so far studies almost always say reduced cancer risk. Plus those on hormones get cancer left and right whereas it's a rare topic in herbal forums.
In terms of diet I have a thread there with lots of nutritional things among other info. The nutritional stuff is not hormonal. I bet she'll respond well with or without it, but they're super healthy and certainly don't hurt. What might be nice would be flavanoids to strengthen blood vessels and possibly make her less veiny. They're in oranges and fresh colorful or dark green veggies. Not that it's a big deal. But having good blood flow without leaks never hurts.