Please tell me if i'm overreacting with my boyfriend, i'm so confused?

Me and my boyfriend have been together for about 10 months and i'm 18 and he's 21. Anyway, his good friend is a complete player and very sly but he always seemed nice to my face. About a month ago I got a call from my boyfriend and he told me that his friend had seen me kissing a guy outside of a club and demanded to know the truth. I have NEVER cheated on my bf and at first I thought that his friend must have seen someone who looked like me and might have been mistaken so i rang his friend and asked him why he said that and he lied and said that I had had a 10 minute conversation with him just before I 'cheated' so it was definitely me, he wasn't mistaken he was lying, I could tell by his smug voice. He then went on to call me a slut and a whore. My boyfriend believed me after I reassured him and told me just how angry he was at his friend for lying and said they weren't friends anymore. Now, a month after the incident I saw pictures of this 'friend's' birthday party on facebook and found out my bf had gone behind my back to his party!! Why would he want to hang around with someone who lies to him and trys to break us up??? I feel so disrespected after being called a whore etc. I want a bf that defends me. I'm so angry I don't know if the damage is already done. What shall I do? Btw my bf met me before this friend so it's not even like a close childhood friend.Help?? And yes, I repeat the friend definitely didn't see me cheating or he wasn't mistaken he was LYING.