R.R.
I agree with Kartrina & Vanessa. My 2 daughters share a room & I often have to put the elder down for her nap after the younger has fallen asleep. It can be hard starting a new routine, expect that--just don't give up. It might take a lot of going back up to their room to settle them back down, if they decide to chatter or play. It did with my youngest who's been in a big bed since 18 months. Try making your friend's son a "special spot" in your son's room--something he will connect with/take ownership of (a special comforter, pillow, stuffed animal) and will begin to associate with the new naptime routine.