Well homeowner stuffed up. He shouldn't have wired it up in the first place.
Who knows what other hidden secrets are waiting for you behind the walls.
I would go for capping or trunking on the wall from a healthy supply. These cappings can be painted in the same colour as the walls.
Otherwise, open up the walls and do it the correct way. It will teach the customer a lesson or just walk away from it,
IMHO.
