First off, give yourself a pat on the back for bringing the lad up right.
Second, wait & see for a day or two...maybe your son's response will bring about a good resolution. What happens next kind of depends on what happens. I wouldn't personally be in any rush to try to resolve it: you've heard an anecdotal story at second hand, what if you 'tell her the truth' and it turns out to be not entirely true, and she takes it the wrong way. I'd want to be very sure of my ground - and my relationship with the mum - before taking it any further. Think also about how whatever you do will play out in school, in terms of your lad, her lad and the wider community. You don't, eg, want your son getting a reputation as a snitch.
Definitely my initial response would be to avoid rushing into anything. More broadly, think about what end result you'd like to see, and what - if anything - you can do to help that come about.
Not an easy one. Good luck.