Ok... assuming this is legit; what you need to find out is where that building is......like Rictor said, you'd need the starting point, the driving distance & possibly an estimated speed (maybe guess from the road bumpiness or noises of cars outside), what kind of car/etc she's transported in, if there are any landmarks on the way or visible from the building, and exactly what this building looks like. I'd be surprised if it was a former hospital in the middle of nowhere.
Also, where & what is her brother doing through all of this? If he's legally responsible, he has a duty to go to the police over it - not only that, he can act as a co-oberative witness to her.
Finally, if you get here phone number, you can sometimes get an address off of google (if it's a listed number, of course); if you're concerned enough to drive over and check, then you can make sure her story is to scratch.... and at the same time, if she seems legit you can even offer to hang around and see if she's telling the truth. The latter is well dodgy an option, though - you'd need to effectively run a stakeout, which you'd only do if you were absolutely convinced (and if she's telling the truth, that could also put you under risk).