I think that you should be as helpful as possible, while not taking their decision. A reply might be:
"The cloudbase is 800', and the clubs certainly haven't got their aircraft out of the hangar. According to the TAFs of the nearby Megaport the cloudbase is expected to improve later, but in my experience, because we are the other side of the hills from them, our conditions can be significantly different from theirs."
That way the pilot doesn't feel short-changed, you have made a slight, but non-directive, contribution towards his decision to cancel, and you have covered your posterior.
JMO
W