Without trying to sound harsh, you have asked for advice and you have it, so what are you going to do about it?
The work is some of the worst I have seen. What he has done is litterally criminal in terms of Notifiable work, so you really need to get onto Trading Standards and Building Control. What has been done is a death trap by no uncertain terms and there would be risk of electric shock and/or fire. This installation will definately invalidate your cousins home insurance. What he has done is not your problem as such, it is his, so you need to act. Luckily for your cousin, you know it is an appauling installation so have disconneced it.
I have heard stories of guys being locked up for sub-standard work and this sort of stuff is what puts them there.
Do everyone a favour mate, including him, and report it to the authorities. He needs to be stopped before he does cause an injury or death or fire. Can you forgive yourself if you read something in the paper about someone being killed by substandard work? I couldn't, knowing I hadn't reported it.