Huh, despite all the "legal" advice on DR1, the guy may not be responsible at all I believe he said there was no agreement or contract so who is to say what he could or could not do with the borrowed car (If I lend someone my car (It is fully insured so this is hypothetical) and she returns with a nice dent in side, sure she should offer to pay but I lack any means to force payment.
Maybe he mentioned he wanted to use it in a demolition derby and the owner said OK or just smiled, not knowing (Hard to figure how he can place a "mechanics lien" on a car other than the one in question but it is the DR)
In reality, pay the thing