Is this the right "red flag"?

So i've known my online friend for two years. When we met, we were just friends, but had a huge crush on eachother. Only difference was, I kept putting him down. My friend dated about 4 girls in the same year, with 2 girls being just a day apart before he started dating him. He finally found this girl, dated her for 8 months, and they broke up. He told me she tried to push me away from him, but he started to grow bigger feelings for me while he was dating this girl. Now, this seems like a huge red flag, and if I was that girl he dated for 8 months and my boyfriend liked someone else while dating me, I would feel like shit. We have casual conversations, with is occasional flirty remarks, but I have no interest in dating him, and I've told him I don't want an online relationship. Any advice?