I'm far from an expert here but I thought it was cheating 101 for married people to not add their side pieces on Facebook. I've read stories of people getting secret second phones to keep their spouses from finding out they're cheating etc. Just from the guy's point of view, being married with kids, it would seem pretty reckless to suddenly add some girl you play games with on Facebook and give her your phone number with the specific intention of banging her and not expect your wife to catch on. Although maybe he's in an open marriage like apparently a lot of people are these days so.....
I don't know man, I guess the best thing you can hope for is that your girl is honest with you. But I doubt she would expect this guy to leave his family for her so I really don't think she's after anything serious with him at least. She could very well have a crush on him (or not!) but the guy has a lot to lose if it's not an open marriage so I wouldn't be so sure that anything's going to happen. And even if your girl is crushing on this dude she could very well have, you know, some morals and shit and realize that going after a married dude is super weak and never act on it.
If you accuse her of something and nothing's happening, maybe not even a crush, then it's all over. In this case you wouldn't be accusing her of just having sex with someone else while you're on a break (or whatever your situation is atm) but also of being a homewrecker which is super hurtful if not true so tread lightly. Better focus on yourself and hope and believe she is honest with you. She wouldn't have left the door open for you guys to get back together if she was over you.