Relation ship advice 26M and 25F

I NEED Advice

So I (26F) in relationship with my partner (27M). We recently got engaged. And we used to work at the same place before I left for something better. This new workplace is safer, this is a security company.

So my partner has always been honest. And I noticed while working together, they would flirt with another coworker 45M (at first platonically) and the coworker would flirt back. I guess since I left, the other coworker would be the only person they would talk to (due to the workplace being a toxic envornment). I saw it all happen, I was jealous and worried about an eventual breakup. And they ghosted me for a bit or distanced themselves a bit.

And now they come to me and say that they caught feelings for this other coworker. And I got information that the other coworker might like them back.

But my fiance hasn’t done anything with them. And cried that this stupid crush happened. And that the coworker is married with a family. But they want to work it out with me. I genuinely love this person and see a future with them.

So the question Is, should THEY HAVE TO CUT OFF the coworker? Even if they’re the only person in the workplace that they talk to?

We’re planning on getting couples counseling when we can.

Thank you for the words of advice!