So him stealing from people in the past is true? What are the other things your bf wanted to tell you about him? If it we're me I'd want to know, if he's admitted to stealing in the past and by coincidence that very night he was alone her money went missing? Alarm bells would be ringing.
Is there any reason they would make it up? That would be very cruel and manipulative.
I guess you know and trust your bf more than this guy that you don't know much about. I wouldn't want to lose a friendship over him, and usually your bf knows whether a guy is right or wrong for their bf (imo) if she's doubtful I'd listen carefully.
So are you willing to believe a guy you just met over your bf who you know and trust and is looking out for you?
In the meantime, see this guy outside the home if you want to pursue it and get to know him for yourself, I'm hoping they're wrong about him and he really is an honest, great guy.