I'm not biased towards the older woman at all. Infact, I'd wager that I'm closer to your age than hers. The difference is, I can look at his behaviour indifferently.
Dehumanise it.
He has person A, her. Person A was sleeping with him, she thought they were embarking on a relationship. It may be true that her marriage was struggling anyway or she may have thought he was the love of her life, and she broke up her relationship to be with him. They used to talk a lot. I'd bet that, regardless of whether he intended to or whether he remembers, he did give her the impression that he liked her. Her was sleeping with her!
Person B comes along, and the man is sleeping with them both. He's texting them both. At this point, Person A probably feels him pulling away and puts more effort in. For at least a month, he kept both of you, like puppets on strings.
Then he left Person A, who has caused a lot of chaos in her private life, for Person B, who was presumably a bit more straight-forward. He's stopped seeing, talking to and sleeping with Person A. This is going to feel like a big break up for her, she's going to be angry and hurt and upset. She's probably going to want to talk to him and get some answers and closure.
If you ignore this, she will slowly heal and leave him alone. Sooner or later, dignity and real-life kick in, and it will for her. So you're best option is to totally ignore it and see it as part and parcel of starting your relationship before he left the old one.
It comes across that you might be a bit worried that he's actually still weighing up whether he should be with you or her, so you want to warn her off so that he doesn't really have a choice. That won't work. Logically, you know that - she's not going to listen to the person who stole her man, and even if she did, there'd be plenty of others who don't mind sleeping with someone with a girlfriend. To be blunt, you did the same thing. So you have to trust him, both with her and with the general public. If you don't have that trust, either because of the overlap or just as a general instinct, listen to that. It's not her that's the problem - I'd sympathise a bit with her, because her position is horrible, whomevers fault it is that she's there - he is the problem.