i'm a 36 DD & i know your gf's pain very well! when i work out, i wear a regular bra with a sports bra over it. also, the quality of bra a girl wears on a normal day can also make her appear smaller. i always wanted to wear the cute bras but one day i ran out of laundry so i wore a "granny bra". i ran into an old guy friend who immediately asked if i had gotten a reduction.
also, i work for an insurance company & most insurances will pay for reduction as long as you can prove medical necessity. one rumor i heard is that as long as you go down at least 2 cups sizes its covered.