It's on his left forefinger, on the skin edging the top of his fingernail
. It looks like it could be a wart, but is painful. He has tried doing all sorts of (horrendously inappropriate) things to it - like freezing it off, sandpapering it, and cutting what seems to be a growth of dead skin, to no avail.
Our local GP has been less than helpful, attributing it to "something foreign under the skin", and doing nothing about it.