Is this betrayal? I feel guilty but dutiful!!!?

I had this dramatic experience last week and from which I still didn't come back to normalcy. I have a friend who recently turned unlawful (drugs,gangs etc.).The problem is since Im in law enforcement I have the duty of protecting the society from him. Anyway I couldn't do it coz of my friendship with him.We had lots of arguments and it ended our valuabe bond.Last week his gf invited me to her place to discuss with her about this and there she told me that she has given him up and while we were chatting coz of our chemistry we kissed each other!!! I was guilty but i too felt that i had fallen for her and she too called me and told that she loves me.Now the time has come for me to take some action against him but I can't do it as my conscience doesn't allow me to catch him...but pls let me know if I should do the right thing or just compromise.Even his ex says that I should do the right thing!!! Pls reply...