The constant vaginal bleeding isnt due to pregnancy or miscarriage if she is taking the pill. The pill affects a girl's cycle and can often cause bleeding at unexpected times. I have had so many friends who have bled for over a month solidly due to taking birth control. Especiallyl if she's had a break from it, or they've just started taking it and so on. If you're are concerned though, a visit to the doctor would be good, they can explain it properly and maybe do an exam to check everythings in working order