I'm not sure why people are having trouble grasping that. I get what you're saying. People who pray to false idols are literally praying to nothing. They aren't real, they don't actually exist.
However, the God of the Bible is real, He does exist. So does He actually hear the prayers of your friends who are praying to false Gods and then honor them because they are selfless?
Archivist, I'm curious what you think John 9:31 means and how it applies to your question -
"We know that God does not hear sinners; but if anyone is God-fearing and does His will, He hears him. "
Personally, I think at the end of the day God always acts in such a way so as to bring Himself the most glory. If that means answering the prayers of a non-Christian, then I'm certainly not going to speak against that. But I would think that is the exception, not the general way of things.