US citizen coming back to US

[immihelp]

I have a question, a friend of mine is a citizen of the US that moved to Nigeria 26 years ago, what kind of visa or passport does he need to get to come back to the US to live , not to visit he is not an immigrant, i dont know the process for him to get here or what paperwork he will need , please send me any information if you can.

Thank you or your cooperation

[immihelp]

If he is US citizen, he can come back to US at any time.