My immediate thoughts regarding the example that you cited about how your friend parents: Talk about it. If I have a friend who does something that I fundamentally disagree with, I can say that I don't with agree or like it. After all, we ARE talking about it, right? At that time--or any other, if we're friends--I can ask my friend to share his/her perspective on what's happening. I might say something like, "I already have my ideas of how I think that that should look, and I must say that I can't get inside your head on this one. How are you drawing your conclusions?" Regarding your friend..."What makes you decide when you need a break from your kids? I sometimes feel overwhelmed, but I couldn't imagine sending them away for that long. How do you come to start seeing that as an option?"
I think that when you let it bother you so much without trying to understand what's behind it, then you are making it all about you and showing that the friendship is only valuable when it reflects what you believe in. You are actually only interested in being friends with yourself and leave no room for a second person to have a separate identity.
I don't have those type of deal-breakers in my friendships. I play it by ear and let the relationship grow me in time.
If I had a friend who started dating a married man, my concern would not be his marriage. My concern would be why my friend sees him as a viable option. If the relationship led to marriage between them, I would caution her to be self-aware and to have the difficult conversations with him regarding why he saw getting started in that way was a viable option for him and what type of responsibility he takes for the demise of his marriage and what he plans to do to avoid that in the future.
I believe that we are each on a journey that no one else can travel for us. Our relationships are supposed to grow us, and not always in the directions of our pre-conceived hopes. In order for us to learn compassion, we have to be shown that everything is not so cut and dried and aligned with certain hard and fast conclusions that we've drawn, especially if from a distance.
I can't think of anything that my friends can do to lose my friendship. I adjust boundaries all the time, though.