Yes because he is now a US Citizen and he is travelling with US Passport. Upon your return back to the US, US Immigration officers inspect your US passport to check if it is valid. They don’t ask many questions to US citizens unless they suspect criminal or suspect you obtained your status through fraud or think you traveled to a country they consider a terrorist country or your US passport is invalid. If your US Passport is ok, they let you pass the custom and immigration since you are a US citizen and US is your home. Immigration officers do not deport US citizens since US is their home.
However, this could also trigger immigration officers looking deeper to check whether you lied on your application (obtained your status through fraud). think carefully and take your own risk if you decide to travel to a country you seeked to be protected from. talk to your attorney before travelling.