They both openly fancy each other and have made it clear they'd do something about it if he weren't with you
Imagine they're alone on a night out, drunk, and she leans in to kiss him. What do you genuinely think would happen? The likelihood is he would cheat, right? But EVEN if not, I am pretty sure it would go down something like this:
He says no but there's a whole lot of sexual tension and lip biting and "oh we can't... it'd be really bad... I really wish we could". They talk about the things they'd do to each other if it wasn't so wrong. They spend the night making "fuck me" eyes at each other, and go home and think about each other while they masturbate.
On Monday morning, they do this whole faux-awkward giddy apology to each other, confirming that although it would indeed be the best sex ever, they mustn't ever let it happen, and they were right not to give into temptation. They both applaud themselves for such self-control and he genuinely believes he deserves a medal and you're very lucky to have him.
This happens every single time they go on a night out. They find more and more reasons to be alone, drunk together, while things keep "nearly" happening. At work, she makes tantalising comments to him on purpose that she insists are innocent and blames his dirty mind. They're flirting constantly to the point all of their colleagues are aware of it.
They get closer and any time he has an argument with you, he thinks about the idealised fantasy with her, because they only know the good parts of each other. It's likely he'll start confiding in her.
By the time they do actually cross a physical line, they're both completely convinced that they really tried their best to stop it, but they fell in love and never meant it to happen and never meant you to get hurt.
I've seen it play out so many times.
It's a no from me.
I think "it's normal to find people attractive" gets stretched a bit too far. Normal is walking down the street and having a fleeting thought that someone is attractive. Normal is thinking that a colleague at work is good-looking, even though you hardly know them and never think of them other than when you need to ask them why their spreadsheet makes no sense.
I really think these situations, where two people have a close, intimate friendship and clearly fancy each other is not normal or acceptable. If you fancy someone to the point you think about them when they're not around, you feel giddy around them, want to find a reason to talk about them all the time, like a teenager? It's not okay in a relationship and needs shutting down right away.
I would genuinely expect someone to move jobs/move teams and block this person if they'd gotten themselves in this far. If not possible, I'd expect some iron boundaries and absolutely zero interaction outside of professional requirements.