What's the reasonable and fair thing to do when you were in the possession of someone else's car, and it was stolen?

My friends 20 year old son was driving her exhusbands car and went to a gym to play basketball; while playing, someone stole his keys and ultimately the car. The car was uninsured and he is still making monthly payments.

Please provide ideas on what would be the fair thing for her to do; no commentary needed on the insurance issue or other observations.