You might ask your friends to consider what is said about the efficacy of the prayer of a righteous man in James 5:16-18:
The prayer of a righteous man has great power in its effects. 17Elijah was a man of like nature with ourselves and he prayed fervently that it might not rain, and for three years and six months it did not rain on the earth. 18Then he prayed again and the heaven gave rain, and the earth brought forth its fruit.
You can bet that during those three and a half years many people prayed for the drought to end yet it was the prayer of righteous Elijah that was efficacious.
They might also consider what is said about the efficacy of the prayer of righteous Job in Job 42:7-9:
7After the LORD had spoken these words to Job, the LORD said to Eliphaz the Temanite: “My wrath is kindled against you and against your two friends; for you have not spoken of me what is right, as my servant Job has. 8Now therefore take seven bulls and seven rams, and go to my servant Job, and offer up for yourselves a burnt offering; and my servant Job shall pray for you, for I will accept his prayer not to deal with you according to your folly; for you have not spoken of me what is right, as my servant Job has.” 9So Eliphaz the Temanite and Bildad the Shuhite and Zophar the Naamathite went and did what the LORD had told them; and the LORD accepted Job’s prayer.
The above passage suggests that the prayer of righteous Job was more acceptable to the LORD, more efficacious, than the prayers of Eliphaz, Bildad, and Zophar or why involve Job at all?
Then ask them whether or not the saints in heaven, where there is no sin, are more righteous than people on earth?