Legal Question in Criminal Law in California

The U.S. won't prosecute him for violating another country's laws while he was in that country. But the other country might prosecute, and it might ask the U.S. to extradite him. I would need a lot more information before I could say how likely that is, or what would likely happen.

Note that viewing child porn is not just illegal in this gentleman's country; it's also illegal in the United States. In some circumstances, the U.S. can prosecute people for things they do in other countries, especially if those activities have an impact in this country.

It's also possible for a different country to prosecute him, especially if his actions had an impact there -- for example, if the site he was looking at was in that country, or if that's where the porn was made. Whether he could be prosecuted will depend on that country's laws.

How likely is it that any of these things will happen? I have no idea. The answer depends on many factors, including how long ago this happened, what evidence is available, which country he was from, the extent of his prior contacts with the U.S., etc. The specifics of what happened will also matter; violating child porn laws is always bad, but some violations are worse than others.

The gentleman should contact a local attorney if he wants more concrete guidance about his prospects. He may also want to speak with a lawyer from his own country.