I'm being optimistic and thinking it sounds like she might have been starting to catch feelings for him and this was a wake-up call that she's getting dangerously close to ruining her marriage. Hopefully this will serve to snap her out of it and she'll keep strong boundaries and be more open and up-front with you.
That being said, if she was, it might indicate some communication issues in your relationship (clearly, since you came to us before addressing it with her) that maybe you both aren't even aware of, so I would also get a few counseling sessions. I've always thought therapy is like a tune-up. Your car will still drive without the best maintenance but if you want it to perform at its peak and last forever, you regularly do maintenance on it. Same with relationships, you can often get out of sync so slowly that you don't realize until it's a problem. It never hurts, and if you don't have any issues, then no problem.
Good luck, keep those lines of honest communication open.