Praying

God discerns the hearts of people perfectly. There are doctrinal truths about appropriate prayer, but God is not a legalist, He is full of mercy and compassion for His creation. Additionally, He honors us in our weakness.

That being said, if someone is purposefully praying to another so-called "god" he is not going to honor that prayer. Demons can respond to such worship, which is why pre-believers should not reject the truth about Jesus. Christians should pray to Father God in the name of Jesus' because it's by Jesus' sacrifice on the cross that gains us access to Father God as He is holy and the work of the cross provides us with Jesus' righteousness (we wear it like a robe).

But let's say there is someone in their weakness that cries out to "God" and doesn't really even understand the gospel yet, but is simply sincerely seeking to see if there really is a God, He reserves the right to respond to that heart. And I believe He has saved millions down the ages by this, though it's not His first plan for drawing people to Him.

You should keep praying for friends and family. You have no idea how powerful your prayers are in fighting back the enemy from them. Trust God, He can work miracles in their lives and hearts.

Some good thoughts!

It is good to pray for others, no matter who they are.

I don't think it's wise for anyone to judge whether God will or will not "listen to" any given person's prayers.

There are no limitations on what God knows. He knows our every thought so, of course, he hears prayers. His response may not be what the person praying asked for simply because God knows so much about a situation than we could and wants the best for us. See Psalm 139..first few verses.