I'm going to ask my husband to stay away from a co-worker
How have you been affected by his relationship with his co-worker?
I've seen him flirting with her, which is absurd to me
Flirting is something that can go too far, but it is okay if they are just friends. If you demand he stay away from her, he might just feel like he needs to do the opposite. Then you'll really have something to worry about, so you might just not try to control what he does for the sake of your marriage.
I think he is already cheating.
If you suspect you should try to find out if it is true before you accuse him. If you blow up you could damage your relationship  and he might not be able to trust you.