Just a normal 1040, but what is on the 1040 depends on what you mean by foreign income.

It is possible the other country imposes income tax on the income if it was earned while in the foreign country, even if the income came from the US, in which case the Foreign Tax Credit may apply.

It is possible the taxpayer could qualify for the Foreign Earned Income Exclusion if the taxpayer's "tax home" and "abode" were in the foreign country, even if the income came from the US.

Check if your client is subject to FBAR and FATCA.