What can I say to him without causing a huge fight?

So I have a really close male friend. We have been best friends for 10 years. He has a girlfriend back in his country and they have been together for a year and a half. He said that he loves her very much but I noticed that he doesn't really talk that nice about her. Like he never says that he's looking forward to seeing her or that he misses her or anything. But he says a lot that he loves her. He kinda Moans about her too saying that she annoys him too. But he confessed too me yesterday that he cheated on her 9 times with the same girl. He showed me texts with the other girl and he sends this girl heart faces and tells her that he loves spending time with her and that he can be himself around her.

I was absolutely shocked and I didn't know what to say. I wanted to slap him but I just told him that I don't believe that he loves her and he got angry and said that sometimes people make mistakes. 9 times is not a f****** mistake though!!! Its a decision by now. I need to say something and I'm not staying out of it either. I feel very bad for his girlfriend. What do you think I should say to him? I want to tell him to break up with her because I want him to be happy and he's hardly happy with her if he's cheated 9 times with another girl. Anyway, how should I break it to him?
What can I say to him without causing a huge fight?
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