Three years ago, I sold my parent's home to an RE investor.  He paid $3500 upfront and was to pay an additional amount at the end of this month.  The home was in terrible condition (interior hoarding) but he also ran into some additional costs he wasn't bargaining for, ie the front brick facade was cracking so he had to replace with siding (not a large area) and this month he had to replace the entire septic system.  

Now he is asking me to work  with him on the payment that is due.  I know times are tough, etc., and I don't want to be a jerk, but I am also not sure that my mother should take another hit to her finances.  As RE investors,  doesn't the bad come with the good?  Isn't the risk on the investor?

He and his mother are very nice but now they are telling me that this house cost them way more than they expected and that some of their tennants in other properties aren't paying...Is that my (my mom's) problem?  

I just need advice from some RE investors.  How do you guys handle this stuff?  I know enough about RE investing that if it were me, I wouldn't expect the seller to make concessions.  Thank you to anyone who replies.