There is something between doing it out of the goodness of your heart and milking them for all they're worth though. He could reasonably ask that they pay his legal fees and compensate him for his time dealing with this. (A couple hundred an hour, say, or whatever he feels is reasonable.) Then choose a good IP attorney to look over it. That's based on the fact that he says he's on good terms with the previous employer and would like to help them out.

This is the most reasonable solution and I hope the OP chooses it.

Some times it can behoove one to keep a relationship amicable and not be hostile or hold a grudge.