I would play on the fact that if they want the new sale then they will try their hardest to get him to trade in. At the end of the day the cost of tyres comes from a totally different budget and i don't think they will be that fussed. I am a hard negotiator and i fine pick all finance quotations and make them work for the new sale. The car should have a better future guaranteed value trading it in 6 months early.
I think he is in good favour. Just need to be hard with them.
First get the quotation and see how it fairs. If the deal is not good then change later.
Only thing is even if its traded in he will still have to wait for the new car, which might not work anyway.