My Boyfriend's Mom's Boyfriend is Cheating

Okay talk about a tongue twister but I genuinely do not know what to do or say in this situation.

I don't want anybody identifying me so I'm going to try and be as vague as possible while still giving everyone a clear picture of what's going on here.

Time for some background.

My boyfriend's parents are divorced and have been since I believe elementary school. (Still a sore subject, tried asking him about it early on but didn't go to well so I try not to press.) As far as I am aware, the family is very functional, and I believe that their mom and dad are cordial, even friendly.

I have been dating my boyfriend for the past two years and we have done all big couple stuff, staying over dinner, meeting the family, all of it. We are both the same age in high school, seniors this year.

The mom is a genuine sweetheart. I mean this woman has truly welcomed me with open arms, been kind, been respectful, been an angel. I am genuinely so happy that I have her as a "mother in law."

Her boyfriend is also pretty cool but I've only met him a couple of times, however, he seems genuinely down to earth and they seem happy together. He always plays nice with my boyfriend's dad too what can I say?

Now time for where there are issues.

I went to the a pretty public place with my family. While I was there I saw the boyfriend with a woman who was honestly the complete opposite of my "mil." They were walking together, hand in hand, looking all lovey dovey while he was holding all of her bags.

It could not be a sister, aunt, mom or whatever. I swear to you that. Honestly, at this point I was in pure shock that I kind of just stared. Part of me didn't want to believe that I was seeing this or was really really hoping it was his doppelganger.

So I stared shocked and I guess he felt my gaze because he looked at me from where I was. He looked at me, I looked at him, and we shared a look that 'oh shit' look. He kept walking away and I just stayed there confused, dazed, scared. I don't know pretty much every unpleasant emotion you can feel.

This has been killing me. Killing me!! I haven't been able to face my boyfriend without going a little bit insane. Of course I keep it under cover for the most part but don't think I can keep this up for too long.

What do I do? I have no evidence. I have no proof. I wish I would have taken a photo but of course I was too shocked to even try at the moment.

Who sends a "hey girly," text to their MOTHER IN LAW????

So, with a lack of resources to turn to I turn to you reddit.

Should I just let this go? Forget it happened? Tell my boyfriend? Tell the mom? Honestly, I just needed to get this off of my chest because it's been weighing me down like you would not believe.