He must be in the U.S. to apply for citizenship. Since he has a reentry permit (I think that's what you are referring to with the 2 year travel permit) he is not in jeopardy of losing his residence, but he will have to be in the U.S. and meet the physical presence requirement before he can apply.
OOPS ... a single absence from the US of more than 6 months RESETS the residence clock, unless a N-470 was filed.
Your friend needs to get back 'home' to the US so that they don't try to cancel his residence.
Just because he has a 2 year permit, it doesn't mean he can stay away for a full 2 years on one trip.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
If he stays outside the US for one year or more, he will lose his ability to seek naturalization for four years and one day.
(213) 394-4554 x0 Mr. Shusterman is a former INS Trial Attorney (1976-82) with over 35 years of immigration experience. His response to your question is general in nature, as not all the facts are known to him. You should retain an attorney experienced in immigration law to review all the facts in your case in order to receive advice specific to your case. Mr. Shusterman's statement above does not create an attorney/client relationship.
I agree with my colleagues. You can find more information at uscis.gov.
973-984-0800. Please be advised my answers to questions does not constitute legal advise and you should not rely on it, due to the fact that we have never met, I have not been aprised of the facts in you case nor have I reviewed any documents.