At the end of the day, what does his offer to buy say? People put in all sorts of things in there!
Your son and "his brother" (not your son?) rented the house for a period of time, then moved out. Their rental agreement was satisfied when they moved out, and their deposit returned. If "the girl that owns it" is somehow a friend of theirs, with no formal agreement with them, then she's in even LESS of a position to say something was not right when they moved out, which is possibly why they're trying to say they need to fix XYZ.
Are you saying the short-term renter couldn't get THEIR purchase figured out because of some dumb blah-blah?
A loan company that says they don't do less than $50k sounds kinda scammy...like, only loaning to people with less-than-stellar credit. Normally you can get a loan for 80% of the appraisal value, so if you have a huge downpayment, or you're buying a fixer upper, the loan amount could be small.
Having the realtor in the same office as the girl's mom kinda raises some eyebrows...your son's realtor SHOULD have loyalty to him, and legally he MUST do the best job for their client. But, if they're in the same office, that bond may be (unfortunately) stronger than some guy trying to get a small loan. That realtor only makes 3% kwim? On $50k, how hard are they going to work for them? When they've got someone they want to work cooperatively with until the end of time? They're gonna wanna put this deal through as quick and painlessly as possible, not so much that they're shady...but if it was a $300k home, they can afford to spend more time working out the details.