What should I do

This girl (she’s in a relationship btw) went to hs with me and she is married and the other day she was talking to me about how her and her husband was trying an open relationship well it and wanted to link up with me well im not really comfortable with it and to me that’s wrong but at the same time she’s been my friend for going on 8 years how do I tell her gently that I don’t feel comfortable with it without sounding rude or making her upset bc she’s starting to send pictures and idk how to go about it