Well, this "friend" obviously has no respect for you whatsoever. What your friend is doing is wrong on a number of levels. I know for a fact that if a guy was doing that to my girl, i'd want to kick his ass too. You may not want to get your friend in trouble, or put him in danger, but this is serious. You've either got to make it perfectly clear to your friend that it has to stop right now or there will be consequences, or better yet, take action NOW!

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"Experience is the name every one gives to their mistakes."