Do you/others have any pic's from the party with them in that could end up posted to FB?
If you are certain they were more than just friends, that would be a cruel way for her to find out, so:
* If she was a friend, I'd see/call her and mention seeing him there and who was he with?
* If she was an acquaintance, I'd say nothing.
* not suggesting YOU post to FB and tag her, but if there is a possibility someone will, it might be nicer for her to hear this first.