@theresnoonequitelikegrandma
I see both sides of this. And I think you sound like a genuinely lovely, sweet lady. Many women would love a MIL like you. 
However, although you have concerns, it is, of course, not your business (don't mean to sound rude saying that!!!) He is an adult and has to make his own choices in life. And also, you can ASK him to 'untag' himself from the pics with this woman, but you can't really insist.
And although I agree (with a previous poster,) that 'it takes two to make a marriage fail' in some cases; it is not necessarily so in ALL cases. And I am suspicious that this is purely down to your son, and I bet his wife has done nothing wrong.
Sorry to say this about your son, but from what you have said, he sounds like a selfish, self centred immature little shit. I reckon that he and that woman he has CLEARLY been having an affair deserve each other. She knows he is married/with someone, and yet still has this nasty little fling with him. Nasty. They are both assholes, who, as I said, deserve each other.
BUT there is not a lot you can do, except be their for his wife.
My husband's cousin (Pete) was a total shit like this, and 20 years back, he cheated on his G/F of 5 years (Jane - who he lived with,) and dumped her callously, by bringing his NEW WOMAN to a wedding that he and his girlfriend had been invited to. Just rubbing her nose right in it, by telling everyone at the wedding that he and 'Jane' were finished, and this was his new woman. Jane was devastated, and ran crying from the wedding (though she knew deep down that he was cheating.)
Jane stayed friends with Pete's mother, and it REALLY pissed him off. They had gotten very close over 5 years, and his being a bastard was not going to cancel out that friendship. They are still friends now- after all this time, and HE has been through 3 relationships since (lasting 5, 7, and 3 years..... the 7 year one he was married to, but cheated on her with the next one who he spent 3 years with...) What a twat.
So feel free to stay friends with his wife. Fuck him. He is a selfish twat.
Oh and yeah, YANBU to be disappointed in him! Don't cast him out of your life obviously, but let him know you are pissed off with him, and as I said, feel free to stay friends with his wife. It's got sod-all to do with him if you stay friends with her. (If she wants to stay friends with you obviously!) 
And no, you are NOT being 'weird' or nosey or intrusive, and you don't sound like you are 'projecting.!' You have every right to comment, and be disappointed in him. You are his mother FFS! Pay no attention to the posters shooting rude and negative comments at you.
I also doubt if this woman will end up becoming the future 'Mrs' either, as it probably won't be long before he starts doing the dirty on her.
Good luck. Hope it all works out OK. 
@speakout and @tinklylittlelaugh, I am sorry you went through a bad time with your DH, but it is VERY unlikely that what happened to you is happening to the OP's son! I reckon he is just a selfish little shit who wants to have his cake and eat it. I agree with a pp that the 2 of you are projecting..