As in my previous post, the damage will be done, although it may not show at present, but will probably raise its ugly head in 1 week, 1 month or even 1 year, when you will probably have to stick in a second hand engine at a cost of circa £2000.00.
Just making you aware of the potential costs as I do not think that you are grasping the seriousness of the situation.
Alex, can you recommend who he needs to consult and what engine checks need to be done.
Once he has this info he has been given every opportunity to get it fully checked out.
OP, I'd then go to citizens advice for the legal side of it.