Jag har varit i USA två gånger, har inte lämnat tillbaka den "gröna lappen" någon av gångerna, men det har inte varit något problem för mig att komma in igen.
Men det kan tydligen bli problem, hittade denna sida
http://www.usemb.se/Consulate/niv4.html där står det:
Q. I just returned from the United States and discovered that I did not turn in my I-94W Departure Record (usually a green form stapled inside the passport) before departure. What should I do?
A. If you returned home with your Department of Homeland Security (United States Citizenship and Immigration Services (USCIS)) departure record Form I-94 (white) or Form I-94W (green) in your passport, it means that your departure was not recorded properly. It is your responsibility to correct this record. You must provide the United States Citizenship and Immigration Services (USCIS) sufficient information so we can record your timely departure from the United States. This will close out your earlier record of arrival to this country.
If you do not validate a timely departure from the United States, or if you cannot reasonably prove otherwise when you next apply for admission to the United States, the United States Citizenship and Immigration Services (USCIS) may conclude you remained in the United States beyond your authorized stay. If this happens, the next time you apply to enter the United States, your visa may be subject to cancellation or you may be returned immediately to your foreign point of origin.
In particular, visitors who remain beyond their permitted stay in the United States under the Visa Waiver Program cannot reenter the United States in the future without obtaining a visa from a United States Consulate. If this occurs and you arrive at a United States port-of-entry seeking admission under the Visa Waiver Program without a visa, United States immigration officers may order your immediate return to a foreign point of origin.
To validate departure, the United States Citizenship and Immigration Services (USCIS) will consider a variety of information, including, but not limited to:
*Original boarding passes you used to depart the United States
*Photocopies of entry or departure stamps in your passport indicating entry to another country after you departed the United States (you should copy all passport pages that are not completely blank, and include the biographical page containing your photograph.), and
*Photocopies of other supporting evidence, such as:
*Dated pay slips or vouchers from your employer to indicate you worked in another country after you departed the United States
*Dated bank records showing transactions to indicate you were in another country after you left the United States
*School records showing attendance at a school outside the United States to indicate you were in another country after you left the United States
*Dated credit card receipts, showing your name, but, the credit card number deleted, for purchases made after you left the United States to indicate you were in another country after leaving the United States
Your statement will not be acceptable without supporting evidence such as noted above.
You must mail legible copies or original materials where possible. If you send original materials, you should retain a copy. The United States Citizenship and Immigration Services (USCIS) cannot return original materials after processing. To help us understand the situation and correct your records quickly, please include an explanation letter in English. You must send your letter and enclosed information only to the following address:
ACS - United States Citizenship and Immigration Services (USCIS) SBU
P.O. Box 7125
London, KY 40742-7125
USA
Do not mail your departure Form I-94 or supporting information to any United States Consular Section or Embassy, to any other United States Citizenship and Immigration Services (USCIS) office in the United States, or to any address other than the one above. Only at this location are we able to make the necessary corrections to United States Citizenship and Immigration Services (USCIS) records to prevent inconvenience to you in the future.
Lite väl sent för din vän att göra ovanstående kanske, men det kan nog ändå vara bra att ha med uppgifter som visar att man inte stannat i landet längre än de tillåtna 90 dagarna.