LamieRobertson
Not awoke
What should happen is your son makes a claim under his own policy and his insurer will, in turn, attempt to make a recovery from the other diver's insurer. When [if] they get all the money they paid out back, they would refund your son's excess and mark the claim as 'non-fault' (see previous post about having lots of non-fault claims).
If the other insurer disputes liability either completely... "it wasn't our customer"... or claims 'contributory negligence'... "he parked like a tw*t and our client did everything he could but couldn't avoid an accident"... then they might not get all the money back and so the claim stays as 'fault', your son never gets his excess back and, possibly, loses no claims discount.
Emails are very easy to fake and so carry very little weight in the negotiations between insurers (because they hold very little weight in court), so without an actual witness, it is very difficult for your son's insurer to force the driver's insurer to admit liability (even though the driver has!). The only thing that will carry weight is if the driver admitted liability on a claim form (if one was ever completed) or in a phone conversation with either insurer (which will have been recorded).
Re emails...do you mean easy to fake as in producing a copy? What if the original is sat on my email system....surely that's hard to fake?