If you are thinking by controlling him, his actions or where he goes, and who he hangs out with that it will prevent him from doing something that he's not supposed to do .. You are sadly mistaken.
You are not his mother .. you need to let him make his own decisions...and even mistakes. Trying to control him to the point where he doesn't even want to be with you anymore will undermine any love or commitment he may feel for you. He will be a faithful companion because HE has controlled HIS own behaviour...he doesn't need YOU to do it for him.
guys need "guy time" from time to time. I love my wife, but sometimes you just need to hang out with the guys with no females around. we dont want to cause trouble or cheat or even look at other girls...we just want to hang out and enjoy not worring about manners or how we look or how stupid we are acting.
depends whether or not his friend is a guy. if yes, ABSOLUTELY you should be mad! He needs to be with you at all times! lock him in the basement!!! (just in case you cant tell this is dripping with sarcasm: you need to be more laid back, or he WILL leave you. no one likes a clingy girl! chill out, im sure it will be fine. he needs time with his friend, its no big deal) if its a girl, i wouldnt be cool with it.