I think the best way for your friend is to allow him to talk to the priest. The priest will be able to help him to return to the faith again. I don’t think it’s a good idea for him to go with you in the confessional.
Yes, not at all appropriate. Confession is the most private thing I can think of. While I have a tendency to be a bit nosy sometimes in general, even I have absolutely no desire to overhear anyone’s confession; it would be violating something sacred and intensely private. I can’t imagine any priest would allow it anyway. Plus, since your friend has been away from the faith for many years, he has years of sins built up (as any of us do if we stay away from confession), and it could actually discourage him further if he were to hear your shorter confession and think his sins are worse or too bad to be confessed.
It is good that you are concerned for your friend, and I am so happy to hear that he decided to go to RCIA on his own. God is working; keep praying for him!
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