If I were you, I'd take up the advice of the folk here who suggest pursuing the pub for an insurance claim. If they have no insurance, then they risk closure - so even if that's the case, you have a great deal of bargaining power. I'm sure they'd be happier to pay up for a new guitar rather than have you making phone calls...
In the meantime, I would replace the guys guitar for him. With anything you can get for now, so hiring is a good idea. Otherwise, he'll be without an instrument for a long while - you know what insurance companies are like...