He is not if she signed a paper saying that he is not responsible for the damage, loss of property or loss of car if stolen. She has to read the contract again and see what the fine print says. If the fine print or the contract doesn't state anything alike that, then you might have a case to take to court which I would recommend getting a lawyer. Also, you don't need a key to steal a truck.
He may very well be responsible. He might be saying that just so you think he is not. I bet somewhere on his property he has a sign that says something like not responsible for stolen vehicals. What you need to do is speak with a lawyer, and/or take it to small claims court because sometimes just because they say they are not responsible, does not mean it. He may still be liable.
Doesn't sound right. When a mechanic accepts a vehicle for work, in exchange for money, he is responsible for it's security & safety while it's in his care & on his property. He's should have insurance to cover customer issues like this. (What if he had banged up the car, moving in or out of the work bay? Not his fault? I don't think so.) Call your state's insurance commissioner/bureau & ask. They know the laws. (I don't, sorry.)
The way I understand it is .. When a shop takes possession of a customers vehicle, they take responsibility for it ... Because they control its safety and security. Onus is on the shop owner to take precautions to protect a customers property while it is in his possession.
My car was stolen from a garage 3 years ago, they couldn't do anything for me, insurance payed nothing since it was only covered by liability (geico) and now it mysteriously turns up in a storage facility that's employed the original mechanic who had the car and I'm being sued for almost $5000 in storage fees. Total bs, and from what attorney says I'm basically screwed.