It really just depends on the kind of birth control you are on, and what, if any, other medications you take.
I have been on birth control for months now to help my period, which can be BRUTAL when I'm not taking them, and my sex drive is just as high, if not higher than it was when I wasn't taking it. I just now have the added bonus of not having debilitating cramps and feeling a faucet that can't be shut off for days.
Have her talk to her OB/GYN about finding a different BC that doesn't decrease her sex drive. It may take a few months to find one as she may have to try several brands first, but it may be worth it in the end!