You claimed that the day trading sessions were the best attended, and the night ones were less so. Doesn't it stand to reason that if the day sessions are gone, that the night ones will be better attended? (Beggers can't be choosy?!)

Personally, anyone who would cancel a trip based on one trading session is getting nowhere close to the experience they need from the convention. I can't imagine any one thing (or two or three things) which if cancelled would ruin my experience. Sounds to me more like your friend is taking his ball and going home because he's not getting his way.