My two friends are insulting my girlfriend.?

My two friends (or people i thought were my friends) called my Girlfriend munted behind my back. For a bit of context im in highschool and 18. My girlfriend found out what they said and was really upset. One of them has been my friend for 13 years and he said he didnt mean anything by it, and that i was just his opinion. I maintain that the fact that It got back to me was a testament to the fact the he was saying it out loud and being disrespectful. He didnt aplogize, rather he seemed indifferent. I was planning to have something at my house this weekend. Coincidentally my life long friend starts aplogizing to me because he knows he cant come to my house if we are fighting. I turn his apology down. An hour later he sends me a text telling me to look on facebook. The other friend i was talking about posted on my wall calling my girlfriend a "mutt dog". The life long friend said he didnt want any part of it, should i beat the crap out of the fat piece of **** that blatantly insulted my girlfriend?