Don't go to VS. They're the Target of lingerie - not much better than Wal-Mart, but people are willing to pay so much more.
Fit the bra band to your band size - the +2/+4 thing is old school and outdated. The hooks are there to tighten as the elastic wears down. Wires digging into her ribs could be indicative of a few things - the cup is too small, which pushes the wire; the band is too tight which changes the angle of the wire; or her rib cage being different than the body type the bra was designed for.
You mentioned surgery changed her bust - is it a safe assumption she has implants then? It changes the question a bit because implants will require cups that are designed different as augmented busts will be different. If that's the case, she might be better served posting a question herself on a forum for women who have had plastic surgery.
edit: and if she ends up being measured at something like a 28G or 30F it's not the end of the world - a lot of women over react when they see such a big letter. It sounds like she feels she needs to go up a cup, as well as down a band size as well, but that means she has to go "up" two cup sizes because of the way bras work.