A potential legal minefield, CJ (isn't everything, though?)
It's always been understood, in the "trade", that he would be entirely responsible for his own wellbeing.
Today though, one of the many "no win no fee" lawyers might surface in the event of an accident. I can hear the arguments now. You knew of the breach of regulations, thus are complicit.
Unfair of course. In the matter of "Employers Liability", who's to say that you're not, in this case, an Employer? Perhaps a Court might take this view since, by bypassing (costlier) safe working, you benefit from his way of doing the work with reduce costs.
I'm only throwing ideas around. I can't say that I've come across any problem.