Although I have little in the way of experience with regards to this beyond second hand knowledge, I spoke with my Life Love, Redpepper and I believe she will be replying when time permits. She and her husband have dealt with a similar situation recently which should give insight.
I have finally begun to realize the importance of sensitivity with these situations. You don't want to lose friends but also don't want to go down a path out of obligation and not desire. I used to harp about "radical honesty" in poly relationships and in dealing with people "testing the waters" so to speak. I feel bad for my lack of sensitivity in this and have no doubt it was at least partly due to my own fears and insecurities. That very blunt approach to communication is better applied within relationships where there is some security.
Hopefully you will get the advice you need and move forward in your friendship