I'm not sure I understand--he doesn't have any alcohol left? Is he going to be completely out before Friday?
No, we can't predict how anyone will react, but he could call the rehab, or his doctor, explain the situation, and ask what symptoms should prompt a call to 911. Hallucinations, racing heartbeat are a couple that come to mind. I think it's unlikely he's going to be felled by something drastic that comes on so suddenly you would panic before you could call for help.
As for whether he will stay sober forever after rehab, who knows. Some people do, but they tend to be people highly motivated to stay sober. Depends in large part how much he wants to.