Can we assume that the car owner is seeking recompense from your uncle for this? If so, your uncle needs to pass this to his insurance company for them to deal with - again, I'm making the assumption that he charged for this repair and has necessary insurance i.e he wasnt just doing it as a favour. The engine would need to be independently checked, which the insurance company should deal with if they think its worth it and dont just make a settlement with the owner.