I will say corsets *can* be an issue for people with smaller boobs depending on how much you have there to push up because they can squish what you have flat. However if you have a little there that can be pushed up by a good corset then they can be very enhancing and flattering - it's hard to say based on size as sizes can vary depending on proportions, I'm a 30C and don't have enough to push up despite sounding a lot bigger than I am from the size! So you'll have to judge if she has enough to push up.
If you want to play it safe I would look for a corset with structured cups...something like this: http://www.lovehoney.co.uk/product.cfm?p=16661 which prevent the "squishing flat" issue and supports the boobs better.
Or get her an underbust corset that she can wear with a bra she loves and feels confident in.
I will say sweetheart neckline can gape on women with smaller boobs so avoid that (I have the Sweet Streak polka dot corset and it requires pinning to make it fit).
Getting the right size is crucial - my first basque was the wrong size and I felt utterly hideous in it and it really knocked my confidence for a while so get the tape measure out and work it all out.
Also - look at product photos from other members - there are some beautiful women with smaller boobs so you can see how different shapes of corsets suit different shapes of women.
Good luck and happy shopping
Adxx