I'd be more likely to take a partner of 10 years back after a breakup (with no evidence of any serious wrongdoing) than someone I'd been seeing for 5mins anyway, all else being equal.
As for the question of why this woman even mentioned the bloke at work, it is a bit baffling. Could potentially have been a 'cry for help' i.e. sensing something wasn't quite right in the relationship and trying to stir things up to provoke a reaction from the OP. I've heard of this sort of thing before where people in longer term relationships start to feel a bit taken for granted and want to try and evoke some passion from their partner (fighting to keep them), even if it seems slightly counter-intuitive to admit to a wandering eye.
Whether or not the writing is on the wall at that point is another debate of course, but I'd never criticise someone for trying to save a long term relationship especially over something which could be a relatively minor hiccup (seems to be relatively short-lived compared to how long they've been together).