I was afraid of "creating" a friend at first because I thought that my H would think I was no longer into wanting him or the M. At first he tried to give the "good for you" attitude but I could tell even then that it bothered him.
So although they pretend it doesn't matter, it does. To some extent. But you have to maintain the positiveness about it every time H sees you. Always give as little info as possible about your "friend". And believe me, he will inquire.
I struggle so much with the whole "letting him roll right back in" thing. That is the toughest part for me. It is hard not to let it happen.