Should I leave my husband to deal with this?

Sue your in-law. The worst possible outcome is your husband divorcing you. Since you're considering leaving him anyway over this, it doesn't matter. It's your money. Try to get it back since your husband can't bring himself to do it. I think loans made during marriage is considered household money so you don't need your husband to stand in court with you if you have the paperwork to back up the claim.