There is a ton of threads about cheating here. Have you done a search? Here are some goodies that address a similar vein as in the situation you described:
Cheating, once removed
Object of desire in a closed marriage?
Who's responsibility?
Need some advice and support!
Basically, my feeling is that EVEN IF someone's SO is poly and totally honest about everything with their partner, but the SO has to lie and hide and sneak around to have sex with someone who isn't open with
their partner and is cheating, it is STILL lying and cheating across the board by everyone who allows it to continue. And no one wants that dishonesty and deception seeping into their relationship by association. You really cannot trust anything the cheater says while they are still engaged in the deception and betrayal. Number of partners? STI test results? Who knows what to believe? Cheaters have to develop lying as a talent in order to get away with it, so where does it end? So, yes, definitely a deal-breaker for me. I could not be involved with a cheater.