My friend found out I've been flirting with his ex. When should I apologize?

I won't go into details, but the main story is, my friend found out I've been flirting with his ex after I told him I wouldn't (yes, that's not a nice thing to do). Now he's mad at me. The question is, when to apologize? As soon as possible or to wait some days until he cools off? I'm afraid if I wait too long, he would spread bad words about me. Thanks.