DD is an inch smaller than me in both sizes (I am 28 and 33).
Having not actually seen her, this is sort of a shot in the dark:
I'd start out with a 30FF (or G, depending on brand). One size up from that would be a 30G or 30GG depending on brand, and one size down would be a 30F or 30DDD depending on brand. Two sizes down would be a 30DD/E. Yes, I know this is getting more and more convoluted by the sentence. My advice would be to buy just ONE bra style in several sizes. You need something to serve as a gauge. She is pretty reliably a 30...but cup size is highly, highly variable.
30's are available at Nordies online but at least in my store, I could not find any, so if you would like to start there, I would give them a call first before making the hike. If you want to give online shopping a try, I really like Herroom.com. They give a lot of good advice and tips about different bra styles. Bare Necessities might be good too but they don't carry my size so I have no personal experience with them.
Once you have a pretty solid idea of her size and needs, Bravissimo and Figleaves would be a good places to look at, since it seems like she will be wearing full figure bras. The only reason I don't advise them right from the start is because you're so unsure of her size right now, and they are UK based so returns are a pain.