Behavior of a friend?

I don’t know what to do about this situation and looking for some help…please! I have this special person in my life who has recently started seeing a married man. This “special person” and I used to date, became best friends and enjoyed many non-platonic visits. She now lives several hours away from me and does not know many people in her area.

A few months ago she started talking about this guy she met through some mutual activities. He started telling her very intimate things about his relationship with his wife and his up-bringing almost immediately after meeting her. This guy’s wife had a stroke earlier this year in her mid-forties (also his age) which has left her with many after affects. She still lives at home with him and their teenage daughter but apparently goes for therapy several times a week and is able to communicate with little difficulty. He would tell my friend, who is in her mid-twenties, how much this event has changed his life “This is not what I signed up for!” and statements like it. I tried warning her that this guy was sucking her into being a whole lot more than just being his friend…and that she was quite possibly getting played. She supposedly reduced contact with him at that point but I noticed over the last month or so she was becoming increasingly distant from me.

On my most recent visit to her home she told me that she wanted to remain my friend but she was seeing this married man. He called several times during my visit with her that weekend and it was obvious that she was going to see him immediately after dropping me off at the airport. My friend has always been a woman who was very outspoken against affairs of this type. I have pointed that out to her and also questioned the morals and ethics of the guy she is now involved with. I do not understand how he is able to leave his wife and daughter at the curb while they need him the most…and do not understand how my friend can justify her part in this as well!

I am heartbroken for myself but also very upset about my friend. I feel that at this point if I continue to interact with her as we were doing, talking several hours every night on the phone, I will only help support her affair by giving her that piece of a relationship that is missing in most affairs of this type. I also want to scream at her but know that will only push her closer to this guy. I have decided not to be there for her (for the 1st time in the five years I have known her) and to just let her live her own mistake. I feel this is the right thing to do but it is killing me to let this person who is very special to me hang in the breeze. Any advice you can give me would be most appreciated!