Tell your friend not to sign any papers. If the officials try to force, tell them you will talk to your lawyer first. Once you sign, they will deport you immediately. Hire a deportation defense lawyer asap. A minor fault will determine whether he will walk in ktm streets or US street tomorrow.
Your friend should have told the officer he worked under CPT or after dso approval for economic hardship and these officials at POE let go many times. I would say it was mere bad luck of your friend that day at POE. Many who sell beer to minor are coming with no hassles. Anyway, good luck to your fren.