Well, I'd be all in favor of unenforceable
The point is that if FIL didn't pay his bills, they can go after MIL for
her charges. She still doesn't owe for
his. Despite billing statements that are organized by his charges vs. hers, AmEx told us they can't possibly differentiate how much of the revolving charges (and attendant fees, etc.) are hers. We haven't decided yet what to do with them or how much to argue, but I suspect we will just pay her new charges and the revolving debt (about 4K) because I think she used the card much more often than he did, so much of it is probably "hers" anyway.