Well, as suggested, I took the high road and called him and said I had bailed out of my trip because of weather. I asked if he wanted to go out to dinner and he said, "Sure." Which is good.
But he's made plans with this other person. If it were me and it were, indeed innocent, I would have said, "Well, I agreed to meet up with a friend at such & such...wanna come with?" Or "I better call so-and-so and tell her I'll be late/not able to come, etc."
After dinner, who's to say he won't go join her? He has to be somewhere early tomorrow morning in a nearby town, and he stays with his folks who live closer on those nights. So I'd be none the wiser if he opted to go by the "happy hour" and check her out.
Seriously, I know him well enough to know that if this were on the up and up, I'd know about it because he'd tell me.