Problems i'm going through now.

basically i have a friend who has a really flirtatious nature we are good friends and we have a running joke involving winking (yeah dont ask) but basically to an outsider it could seem kind of dodgy. im single and he has a girlfriend hes not exactly faithful but hes a really good friend to me and i barely know his girlfriend as she took an instant disliking to me the first time we met and so i wouldnt really ever say to her that he has cheated. i only see him as a friend and nothing more but hes told me that shes overly jealous and convinced there is something going on. shes a bit of a nutter in all honesty and would probably kick my ass if she had the chance and now she has just added me on facebook i havent accepted yet what should i do!!???