Does one need to show a greencard for citizen app or can they just check ss#/records etc.?
Answer:
USCIS will not make your friend replace his I-551 to apply for citizenship, he will just need his "Alien number" (commonly referred to as A-number) that was recorded on his card.
You should know however that your friend is required to have proof of permanent residency on his person at all times if he is at least 18. Any U.S. Customs and Border Protection Officers, Border Patrol Agents, U.S. Immigration and Customs Enforcement Agents will want to see the actual card if he comes under their scrutiny.
Your friend can save himself a lot of hassle from the police if he just replaces his card. There are a lot of places that deliberately look the other way if they think someone is an alien but there are just as many places that will call ICE or BP to have someone checked out. They won't stop someone just to check but will do it during a field interview or traffic stop, etc.
Also, if your friend leaves the U.S. without it he can be fined $265 upon his return for not having the card on his person. That is if he isn't expeditedly removed as an immigrant without a visa.
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