I'm wondering if you and your friend had a legal agreement drawn up to cover this loan.
I'm not sure what your legal position is, I'm afraid. The only advice I can give you is "never borrow money from a friend, or lend it either". Sounds hard but it's not worth the aggro if things start to go wrong.
Sounds like your "friend" is a bit unreliable anyway if he's hiding money from his wife. I'd be inclined to tell him to "back off" otherwise you'll tell his wife about the hidden money !!