I'm inclined to agree that returning them with no strings attached is the "best" way to go. It seems to me we too often get all hung up on our "rights" as photographers, or auto-mechanics, or whatever, and forget all about our "obligations" as members of the human race. This doesn't appear to be either a career-maker or career-breaker as far as you're concerned and it's apparently causing severe distress in the relationship of the other people involved. Return all the documentation, put the situation to rest once and for all, and consider it a cheaply-paid-for lesson in the art of human relations. You might even feel good about doing it. At the very least you'll know you did something to help somebody out of a difficult situation. The ole cowboy.