Already been said, but I can't emphasise enough the electric testing pen, and hit that home... even if he knows it all and is invincible, just remind him.
I've been zapped before through the carelessness of an electrician, but equally, I was careless not to check it myself... didn't ruin my day, nor do anything other than open the colourful language valve...and it really upset the electrician because of the whole "what could have happened" scenario.
Oh that and a packet of 15mm stop ends incase he has a run in with any of the plumbers I've experienced
Not brand specific, but one of these has been well used by me
https://www.screwfix.com/p/bahco-combination-square-12-300mm-/3870k