Come through means they were there for each other. When one needed help or encouragement the other one was there to be a friend. Subtract means take away or make something less. In this case the fact that they fought all of the time didn’t make their relationship or friendship less that it would have been if they did not fight.

Thanks so much michael

When you "come through" for someone it indicates that you helped them be successful at something or perhaps you rescued them. However, most people would not use the word "subtract" in this sentence. They would use "take away from". I know it is confusing because definition-wise "subtract" and "take away from" technically mean the same thing. But "take away from" is the normal expression.