My friend doesn’t respect his girlfriend. How to make him understand what he’s doing is wrong?

I’ve been friends with this guy for 2 years, we both like each other (We never dated due to a complicated situation so we agreed to just stay friends).

A couple months ago his brother set him up on a date with some girl his wife know.

They’ve been dating for awhile, I don't know his girlfriend and never seen her before, (by the way my friend and I work at the same job)

He’s always been a little flirtatious with me back when he was single. But unfortunately he’s still doing it but not as much.

I walked by him and his friends and spoke, he said hi back, and when I walked away I heard one of his friends in the background say “Nah, turn back around (his name)”. And I turn to go through a door and I happened to glance back in their direction, and I see my friend was staring right at me (He was checking me out which he does when he think I don’t notice)

He admitted to one of his friends that I’m still gorgeous and hot to him and that he still wanna see and spend time with me.

And it’s so messed up, cause why agree to go out with this girl that his brother hooked him up with, but still has huge feelings for me?

If he really wanted me, he should’ve turned her down and worked things out between us instead of trying to stay close to me and saying things to/about me behind her back. He even blush when I talk to him normally about work stuff.

And his girlfriend has no idea about it (Mind you his girlfriend doesn’t work at our job so I have no way of telling her). I wanna tell him it’s not right and I don’t wanna become “the other girl” 😬

And with him staring, it’s one thing to glance, but stare when he has a whole girlfriend is starting to creep me out.
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He’s a nice guy, but I never notice this behavior with him towards other women at the job. It’s only with me. I think he kinda feels bad we’re not together so he wanna stay close but it’s for the wrong reasons
My friend doesn’t respect his girlfriend. How to make him understand what he’s doing is wrong?
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