I don't think the other woman (or man, for that matter) ever has any responsibility when it comes to someone else cheating on their partner.
I would only agree with that if the third person didn't know that the person doing the cheating was in a relationship. If they knew the other person was in a relationship or married and still did it anyway then they are just as responsible and shitty as the cheater.
Camming is different though and a cam model can't be considered in that way as she is just doing her job.