I'll try and help as this is my field of expertise.
Who is your mate's insurance company?
If he's gone into the rear of another vehicle its his fault period.
Ok i'll explain how salvage works - but again it does depend on your insurance company. Generally speaking :-
If the vehicle is a write off, the engineer will give you whats called a 'pre-accident value' i.e the value of your vehicle before the accident. He will also give the vehicle a scrap value i.e how much it is worth to a scrapyard.
Depending on your insurance company you can request that you retain salvage of the vehicle and deduct that from your 'pre-accident value' settlement.
E.g they offer you 500 pounds value, salvage 200. You keep vehicle and get a cheque for 300.
However.....again pending who your insurers are - if the vehicle is deemed unsafe to be put back on the road, some insurers will refuse to give it you back and will scrap it. This will be in the terms of your policy.
Hope this helps.