Information on returning back to the US

I have a friend that lives in Germany. Back in the 60's her family came to the US. She spent around 13 years here, she went through school and even has a US high school diploma. She has her expired green card, a SSN card, old state ID, and other documentation showing that she lived and worked here for those years. She also has her parents documents, and proof they currently draw a Social Security check from the US government.

After a recent visit, she is wanting to move back to the States, and would like to know what has to be done to do so.