For starters all U. S. citizens are required by law to report their global income from all sources. As a tax professional you have a legal and ethical obligation to advise him of this. Failing to advise him properly COULD easily land on your shoulders and negatively impact YOUR license.

Secondly, there is NO foreign Earned Income Credit - there is a foreign earned income EXCLUSION and there is a Foreign Tax Credit. If he qualifies for either he may not have a Federal tax liability, but there's no way to know until you do the returns.

Thirdly, he likely also has a STATE tax return requirement. California is the ONLY state I'm aware of that MAY let you waive out of a state filing IF you've left the country with no intention of returning to California when you come back stateside. Every other state that I've on expects you'll come back so you still have to file in those states. AND some of those states, like Maryland, follow the Federal Foreign Earned Income Exclusion rules, allowing you to exclude the same income from state tax.

But some states, like California, do NOT recognize the Foreign Earned Income Exclusion. Meaning if he is expected to return to California he would have to file a California tax return, get NO Foreign Earned Income Exclusion AND pay tax to California.

Whether he's responsible for SE tax is a factor of his employment status. If he was an EMPLOYEE of the Canadian company he would not be allowed to contribute to U. S. Social Security. If was a part owner in a partnership I THINK he's subject to the same SE tax rules as any other partner. Though I honestly do NOT know for sure how you'd interpret this if it was a Canadian partnership. It is possible he'll qualify and get Social Security from BOTH the U. S. and Canada.