I am divorced and have my green card. In fact it came up for renewal shortly after the divorce and nobody asked. However, I was married for over 20 years.
'If You Are Late In Applying To Remove The Conditions On Residence
If you fail to properly file Form I-751 within the 90-day period before your second anniversary as a conditional resident:
Your conditional resident status will automatically be terminated and we will begin removal proceedings against you
You will receive a notice from us telling you that you have failed to remove the conditions
You will receive a Notice to Appear at a hearing. At the hearing you may review and rebut the evidence against you. You are responsible for proving that you complied with the requirements (we are not responsible for proving that you did not comply with the requirements)
The Form I-751 can be filed after the 90-day period if you can prove in writing to the director of the appropriate Service Center that there was good cause for failing to file the petition on time. The director has the discretion to approve the petition and restore your permanent resident status.'
From NatashaBee's link.
If he didn't apply to remove the conditions then I think your nephew needs to consult a lawyer.
On travel outside the US for green card holders --
'Does travel outside the United States affect my permanent resident status?
Permanent residents are free to travel outside the United States, and temporary or brief travel usually does not affect your permanent resident status. If it is determined, however, that you did not intend to make the United States your permanent home, you will be found to have abandoned your permanent resident status. A general guide used is whether you have been absent from the United States for more than a year. Abandonment may be found to occur in trips of less than a year where it is believed you did not intend to make the United States your permanent residence. While brief trips abroad generally are not problematic, the officer may consider criteria such as whether your intention was to visit abroad only temporarily, whether you maintained U.S. family and community ties, maintained U.S employment, filed U.S. income taxes as a resident, or otherwise established your intention to return to the United States as your permanent home. Other factors that may be considered include whether you maintained a U.S. mailing address, kept U.S. bank accounts and a valid U.S. driver?s license, own property or run a business in the United States, or any other evidence that supports the temporary nature of your absence.'
Has he filed a tax return this year? If he worked outside the US he will still owe taxes and even if he didn't work he should still file.
I really think he needs to see a lawyer.
It doesn't look good if he closed his bank account, etc.
....
If he has no driving record in the US then he will be paying quite a bit for insurance initially. Maybe not as much as he would if he was under 25.