(I'm so cross I typed my reply to this twice and it effing wiped it!)
Anyway, so a briefer version in case it disappears again - I reckon either way your mum will get stressed; if he is uninvited your mum might feel bad for a bit, but if he turns up you and gav are going to want to break his neck so that could cause loads more hassle and stress....
So I would tell him he isn't coming. I have no time for that kind of bullsh*t at all (I only tolerate funny bullsh*t
)