Poacher,....now that is a good term. I like that label.
As for the OP. I agree with SC. It is ok to intervene when it can contaminate your own relationship. Now that obviously offers different perspectives on what 'contamination ' is. I think they key is that you can intervene and state your case with logic. Don`t hand out ultimatums, but be consistant.
I`ve always found a written/typed word, gives people time to read, think, and reflect, rather then feeling defensive in the moment of conversation. ( about such topics)
My husband and I have only every 'intervened' once on each others behalf,..and neither of us forced the others hand. We stated our postion, and left it with the other. Trusting that eventually our partner would 'see the light' so to speak.
We both did. Happily. Without resentment. Probably because it was our own choice, not forced on us.