hm... it would help, I think, if we knew what she switched from/to. have you talked to her about this change? it could have NOTHING to do w/ the meds. is she having a stressful time at work/school/with family? are you two fighting about other things? is she depressed? is she on some other meds? all this can effect sexual libido.
So, the solution to this is: talk to her about it. ask her what's bothering her. explain your concerns. no accusations. no "well you are being mean, 'cuz i'm not getting BJ's!". that won't get you anywhere. good luck.
edit: also, it may just be that this particular pill she's switched to has some negative side effects, and in that case, she should get her tail back to the gyno to switch perscriptions. the doctor should work with her on this issue.