If your spouse cheats and deceives the third party in such a way that the third party does not know they are being used. This lets the third party off the hook. If the spouse badmouthed you to the third party with lies to garner sympathy of the "shitty marriage" this is a mostly also your spouse, but the third party at least know of a marriage, although they think it might be more equivalent to prison.
If your spouse cheats with someone who has full knowledge of the situation, that person takes on another level of responsibility. If there are kids involved, and they know, that's a wee bit less good too. I think there is a lot of grey area here to lay blame. I think its pretty easy to make the case that sometimes the third party is a mega asshole and holds much responsibility, and other times they can actually be an unwitting player pulled into the greater relationship drama.