the person who gave him the keys would be liable. In this case, he owns the airplane so he himself is liable.Haven't been able to get a hold of him yet but I will shortly. I will void the most recent endorsement. I'm not going to call the FSDO but I will let him know that I will if I catch him again, not to rat him him out, but as stated before, just to say what the guy is doing and that I told him that he is breaking the regs and see what they say. We do have an obligation to protect the general public and fellow pilots.
I get what people or trying to say about the taxi thing (no endorsement needed) but I'm not sold on it. That's like saying if some stranger comes in the door I can hand them the keys and let them go taxi around. Who is to blame if they cross an active runway, or crash into another airplane?
You know, this may be a hard concept for some to grasp, but in most cases, the person doing the act is also the one who is liable for that act.