Start by reading the lease agreement very carefully, and understanding what "rights" are afforded them.
Then go to your local government housing department web site. I would think you should be able to find a listing of tenant rights, and/or links to resources. If the local office does not have a web site, they should be listed in the phone book, so call.
You might also want to see if you have a local legal aid society that can provide some answers/information at nominal or no charge.
Has your nephew tried negotiating at all with the new owners? Do it sooner than later. A lot depends on the owner and what plans they have for the property. I would think they might be responsive if say, you requested a somewhat definitive extension period, like two weeks later, etc. If you try to extend the date too far down the road though, you might be less likely to be accommodated. If you do get an extension, be sure to get names, and try to get it in writing. If they won't give it to you in writing, write to them, "confirming our conversation today, May 10, 2006, blah, blah, blah..."). Oh, and you want to be sure that the person extending the move-out is actually authorized to do so...
As far as actual eviction is concerned, to the best of my knowledge (in Cali), it takes some time for an actual eviction to go through. It appears that your nephew has been given a 30-day notice to vacate the premises, but to actually "force" them out (sheriffs/police go to the premises and lock them out) generally takes longer. Proper procedures need to be taken and legal papers filed first. But you really don't want to go down that path. Depending on how it all works out, your nephew's credit could end up adversely affected (if a judgment is recorded against him).
Important: Keep in the mind that the foregoing is my opinion only, based on limited knowledge and real estate experience, and worth every cent you're paying for it. Your nephew really needs to consult with a local attorney and/or related "experts."