If you e-filed, you do not need to mail the 1040 and W-2. Your friends that are nonresident aliens have to include documents that substantiate any income and withholding amounts when they file 1040NR or 1040NR-EZ as nonresident aliens.
The reason for that is because the IRS doesn't allow e-filing for nonresident aliens, so all international students that are nonresident aliens would have to mail their return with the needed documents.
