Please, please, PLEASE find a way, with whatever tact you feel is prudent, to stop him doing that.Basically, my father-in-law comes over now-and-then to do a bit of electrical work. He is a retired electician and likes to use, what I call, a "volt-stick" now and then. BUT, he often forgets his.
I have no idea what he is like in general terms, but as far as electrical work goes he is a doddery and dangerous old fool.
Sorry, but lives are at stake here - yours, his, his daughter's, his grandchildren if there are any.
A volt-stick is no use whatsoever for anything which requires it to be of use.