What moral responsibility does the accessory-to-cheating have?

To clarify, whenever someone cheats, there are three parties: the cheater, the cheatee (the person being cheated on), and the Other Person/accessory to cheating. (I find it interesting that if I said the “other woman” it would be perfectly clear what I was referring to, but “other man” isn’t as obvious.)

If you accept the premise that cheating is wrong, and the cheater is acting immorally by cheating, do you also believe the other party is acting immorally? Let’s assume that the other party has full knowledge of what’s going on. And for another twist, let’s say the other party also is friends with the cheatee (the wronged boyfriend/girlfriend).

How much moral responsibility does the other party have? Any? Why?

For bonus points: why exactly is cheating wrong? From a pragmatic standpoint, is it only wrong if some negative consequences comes from it? (STDs are transmitted, the wronged party finds out, etc.) If there are no consequences, is it still immoral?