Is it worth the risk though? If something does go wrong who will be to blame in this case?
You can't expect the photographer to take the blame when he has no reimbursement to cover his outlay, time and skill set. Plus I doubt that any insurance will be involved since there is no payment changing hands
You only get one chance of capturing the wedding day and for most women (and some men) it means a lot, both parties here have a lot to lose potentially!
Are both parties totally clear on this and have you both got a written agreement in case of issues arising? I like the idea of someone helping out but if I were the photographer I would insist on a written contract before I even got in the car.