J.B.
Well people can sue for whatever they want...whether or not the suit has merit will be for the magistrate or judge to decide.
Perhaps it would have been nice for Dan and Collin to offer to reimburse Bob for their use of the machine once they became aware that there was an issue with it. Seems pretty crappy that two people used this machine free of charge and now that there's an issue are saying "sorry dude you're on your own." Perhaps Dan (who was un questionably authorized to borrow the machine) could go with Bob to a repair shop to get a third-party to tell them what's wrong and how much it will cost to repair. If it's a lot more reasonable than $4K, maybe it's something that Dan and Collin can agree to pay part of regardless of whether or not they know the damage was from them because it would be a decent thing to do given that they got use of the machine free of charge.
Hopefully at the end of the day everyone has learned to only lend or borrow valuable equipment with written agreements or to J. rent from a professional shop instead.