Should I forgive him or just leave him?

Today my backstabbing socalled "friend" told my boyfriend that she likes him (she doesnt she just hates me)...well it turned out that he told her he liked her too?! Well she texted me and told me and even took a picture of the chat she had with him which was thru facebook and I saw it and I couldnt believe he said that. I am so dissapointed and heartbroken right now. I then messeged him and told him what was up with that and he told me he was just joking. Well i told him i was mad at him and never wanted to speak to him again and he got sad and people started asking what was wrong. He told them he was just playing around and he loves me. I gotta say,he is the stupid,immature funny kind of guy. He loves playing around but anyways should I forgive him? He told me he was very sorry and he didnt mean it and he doesnt want me to be hurt and promised he wouldnt do it again.,,,oh and now he hates her? -.- Should i believe him and forgive him or just break up with him?