Yes, you can do that easily. All the foreign airlines are using the new Concourse A and the transit area is well staffed and rarely get busy unless you hit the arrival of the SQ 15/16.

The transit desks are now all located after security, so your friend will need a printout of the e-ticket of his return flight to the US, which will satisfy the security staff. Don't bother leaving immigration/custom! The line can be long at peak hours. Just an e-ticket printout will be sufficient and then you can get your boarding pass at the transit desk on the departure level.

Of course online check in will be great in this case, but even with a home print boarding pass, I will double check with the transit desk/the boarding gate!

Carfield