I don’t think he has necessarily cheated, but I’m certain this isn’t all innocent either, and he’s leading himself subconsciously or knowingly down the path of temptation.
What I imagine or think is likely to have happened...they have gotten close at work, probably just friendly but some mild flirting peppered in, but not so much that he is certain she actually likes him like that. He had probably suggested training/work at his place, have lunch/brew/chat etc. An easy invitation for him to float because under the guise of work she can easily say no, and he has the excuse of just offering as it being easier for her to understand XYZ in person or some such excuse. Maybe he didn’t really expect for her to accept this invite and is perhaps a bit shocked that she did. Younger, attractive, if I were him I wouldn’t presume she was interested.
He’s not mentioned this to you because well In broad terms he is doing nothing wrong really, he’s simply invited a colleague over to get some work done. I think he didn’t mention it because I think he didn’t want to implicate himself or feel guilty, because in the back of his mind the reason why he’s invited her is precisely because he likes her.
I’m not saying his intentions were to cheat, but I think his intentions were less than pure, and perhaps he wanted to see if there was anything there (if he’s in with a chance) when spending time together alone and in person. She probably bought the pot plant to be polite and because he was hosting her.
I think the fact he also chose that day to tell his ex about you is also very telling indeed. I wonder if he did this as a way to keep himself in check, to tell himself that what he is doing is innocent and not in fact what he is actually doing, which is considering the idea of cheating with young pretty colleague. He’s just trying desperately to keep up that narrative that he is a good guy who is getting serious with you, and telling his ex about you is proof of that.
I don’t think he can probably even admit fully to himself the reason why he invited this colleague over, let alone admit it to you. It seems he’s deep into convincing himself that it’s all innocent.
I think this is why your spidey senses are tingling OP, not because he’s cheated but because it feels like he almost intends to, or is at least fantasising about it.
All that is by the by though, because what really matters here is you, and how you feel about things. If what I’ve written is true, is it enough for you to leave him? Or would you rather just hope he never cheats and stay? Is him actually cheating a complete dealbreaker? What are your boundaries OP? Where do you want to draw your red lines, something to think about.
I’m sorry if what I’ve typed out had upset you 