My friend's boyfriend flirts with me?

My friend and I have known each other way longer than she's known her boyfriend. When she isn't around or isn't looking, he flirts with me, and calls me pet names like his "second girlfriend". At first, we all (all three of us) played along and joked like we were a . But he refers to me as baby, sends me vulgar messages, and even pictures of himself! I've told him to cool it, but it keeps happening. I've told my friend but she's in denial. She forgives him and he even once said "What she doesn't know can't hurt her." We all used to be friends, but now I avoid him and he's pestering me to "forgive" him. Am I the one who's wrong?