coulda woulda shoulda

please i need your help i have been married for 6years,my husband cheated on me with my best friend of 22years (she was drunk he was drunk blahblahblah) i have spoken to him about it and i really feel i should leave as i don't know if i can forget {forgive maybe}. last night he called me in the dead of the night and said he gives me the permission to cheat on him with anyone of my choice. he also said it can be his best friend if i want to........ please what do you advice do i leave him? Do I cheat so we are even and that way it would stop lingering on my mind? Or do I stay and hope I can forgive and forget??????

Kat