Interesting.
I broke up with a boyfriend I'd been with for over a year in my early 20s when I found out he'd kissed someone else...
Here are the circumstances though and if they were different, maybe the outcome would have been different: he was abroad on a group holiday all staying in one giant villa (mix of boys and girls who most of them were mutual friends of ours), I was away with family so didn't go. When he got back he confessed that on TWO occasions he'd kissed one of the girls from the group during nights out there. It was one of the girls who I didn't know personally.
I ended it BECAUSE it was a friend of a friend, all of our friends in the villa knew about it (they had all witnessed it) and it happened on more than one occasion.
IF it was with some random girl at a random bar who he'd never bump in to again once he was back home in England, then maybe I'd have forgiven and forgotten.
OP, if it's a work colleague I'd be incredibly upset and suspicious.