If you don't live as you believe, then eventually you believe as you live.
Homosexuality is forbidden in Islam. No one is sinless, yes; but your friend have already developed an idea that he will be forgiven. Anyone, even prophets can't guarantee such a thing. If there will be a forgiving, it will come after a repentance. Repentance doesn't mean just saying the words, making forgive past sins and going on with the same kind of actions. It requires real remorse and struggle for not to do it again.
Allah orders something, your friend say "I am a Muslim" which means "I am going to live according to Allah's orders", then he acts against it and claims that Allah is going to forgive him anyway. Excuse me, but this is seeing Allah as an idiot. In this kind of thought, there is no need to follow Allah's orders. We can kill innocents, we can take their properties, we can steal etc, Allah will forgive anyway.
Whoever says that "I am Muslim", then we have to accept that he is Muslim. This is because we can't know what his heart really carries. A man may not be a Muslim even if he claims. He may say a lie, or he may not aware of it. Yet Allah knows who is Muslim and who is not. Allah is the only authority to decide who will be forgiven and who won't be. Also Allah is the one decides who is going to Heaven and who is going to Hell.
I mean we can't just decide whether a man calling himself Muslim is really a Muslim or not. But there is a clear contradiction here. There is a logical fallacy. I advise your friend to get rid of false assumptions and to decide what he will do according to that.