Worst part is coworker because it could get legal regarding the workplace.
Other than that don't get emotionally involved or allow her to. I have no trust in the word of these women. The only way for me to get through it is to treat it as a booty call and nothing else. I've never been close to a woman's partner that I've been with, but then I've rarely known there was a partner to begin with. So how you handle your respect for this guy is beyond me. I always get justifications such as the partner pursuing other women beforehand, but, given that I'm usually not made aware of the partner to begin with, why would I place any value in that?
If the workplace issue isn't significant and you can maintain distance it's probably alright for a short period. I don't see myself asking a guy permission to bang his wife or banging a guy's wife without his permission. I get that it's a catch-22, but that's just me.