Not really sure demented. If he said he'd pay for it I'd be chuffed. Perhaps the upheaval? Though when we had it done before it wasn't that bad. He also thinks we couldn't fit a bath in but I think it's worth a look at least.
I think YABU, both to your DH and your cousin. I woulnd't dream of having something done to my house that we hadn't discussed and agreed over that isn't to way i wont push until i get my own way of course. Also, if you finally decide to respect the fact that your DH doesn't want this work done, you will have wasted your cousin's time.