Sorry for what you are going tbrough. It sounds very hurtful.

You were very wise not to move out. That would be a step in the direction of dissolving the marriage, which you don't want to do. I knew a guy who's wife wanted him to move out temporarily. He consulted a lawyer. The lawyer told him to absolutely not move out, which could be read as him abandoning the marriage.

Your wife is in an inappropriate friendship with this tennis partner. That's nice that he seems decent, but he's also encouraging something that's inappropriate. (He gives her the code to his gated complex. She stops by now and then. This is not normal. He's meeting some loneliness need of his own through being with her. Something's out of balance in his life too.)

I suppose you can hang back and let this play out. Trouble with that is that your wife is likely to just drift further and further away from you. I don't often suggest this, but I think the two of you might benefit from marriage counseling. Your wife may be reluctant to go. She'll figure no counselor is going to respect what she's doing.