@Russell brown
You gotta do what you gotta do. The agreement and all terms, clauses and small print in that is between you and the client.
Me personally, I wouldn't want to touch it. I'd be peeved someone else got the work. I get the sense this nay if been a time decision rather than not putting faith in you. Someone who has already done a lot if other works for him.
The main situation you got is covering your own backside. Dave makes a very valid point regarding the quartz and UFH. You said yourself no decoupler. Whatever you do to make it look good doesn't get away from the fact it could all fail anyway.
Maybe look into it a bit more and explore options with the client. If it looks likely no amount of patch up will achieve a long term solution then he maybe more inclined to go with a full rip out.
It's a toughie, but only yourself is in the position to make the right call.
Good luck regardless