You can offer to take over the payments only if they agree to allow you to assume the liability but with a significant enough discount, with the alternative being that they can try to collect on someone who is slipping into insolvency with few to no assets of her own. It makes the decision a bit easier on their part because they either have to sink time and effort into someone they might never collect from on the one hand, or take a bit of a haircut but know they'll get paid. It's a no brainer that way. The amounts in question are too low for litigation to really make sense.
If it comes to my wife and I assuming liability, we would just pay it off immediately rather than make payments to this guy (or MIL's bank?). If the guy is willing to be reasonable, then we'll just write him a check to be rid of him. If not, MIL, at 80, as a homeowner with no possessions of particular value or savings the guy can get at, has little to lose with bankruptcy, I think. Unless the guy can garnish her alimony, and I think that would be protected in bankruptcy because she barely makes enough to support herself.