Whew... I dunno, but I SURE would have walked over and made a point to say hello to him and "Oh, where is Janet? Would you mind telling her we said hello! Who is your friend here? I don't believe we have been introduced"
He will feel like such a lowlife that he may possibly end the affair. You can also casually and innocently mention during conversation "Oh, Janet, did Bob mention we ran into him and his friend dining at such-n-such last week? Yeah! we were there for dinner and just had to stop and ask how little Susie is getting along in school"
If you do this innocently enough and with the intent to sound as if you had NO CLUE that anything could POSSIBLY be going on, she will find out what happened without you being a tattle-tale and without anyone being angry at you.
And you will also spare her some of the humiliation she is guaranteed to suffer, so be tactful enough to pretend everything was completely on the level. Everyone knows the truth anyway...