He’s arguing for moral relativism and has not proved it at all.
His sole claim is that accepting Jesus as Lord is apostasy for a Muslim. Since Islam does not recognize the divinity of Jesus and further asserts their Prophet as inspired by God, he is correct that a person believing these things would err in leaving that religion. That is not because these things are wrong in themselves, but because such a person commits apostasy against his beliefs.
Moral law does not require anyone to act in defiance of their beliefs.
I discussed this in post #9.
It is moral to accept Jesus.
Morality does not necessarily require accepting Jesus.
Only those who believe in the divinity of Jesus are required to follow Him per moral law.
Morality does require integrity (i.e. acting in conformity with beliefs). This is the only point. Therefore, if a person believes in Islam, he is required to follow it. If, however, a person believes in Jesus, he must follow Him. Anyone who follows a religion he does not believe violates moral law. This is not subjective.
The same principle is true of civil law. A pedestrian is not required to wear a helmet even though he uses the same road as a motorcyclist. Both must stop at the same traffic light and neither must wear a safety belt. If the motorcyclist dismounts, he may opt to remove his helmet. If the pedestrian mounts a motorcycle, he must don a helmet. If either travels by car, he must wear a safety belt. The law doesn’t change. What the law **requires **changes depending upon the mode of transportation selected.
Even though beliefs are not chosen in the way that a mode of transportation is chosen, the analogy holds. You must act in accordance with your beliefs just as a traveler must act in accordance with his mode of transportation.