Your friend apparently trusts the information that her family provided more than she trusts her own instincts or her own judgment of other people. Maybe she has a history of bad decisions or there is a history of distrust within her family. Alternatively, maybe she wasn't being honest about something. I suspect there's a lot more to this friend than you know and the reason she gave may not be entirely true. That's just a hunch, so take it for what it is.