Looks like a dual-status alien to me.  But it's confusing when you indicate he is "self employed," but has an "employer."  What was his total income for the year?  Just the $12,000, or is that in addition to something paid from Britain?  

Resident Alien at End of Year

You must file Form 1040, U.S. Individual Income Tax Return if you are a dual-status taxpayer who becomes a resident during the year and who is a U.S. resident on the last day of the tax year. Write "Dual-Status Return" across the top of the return. Attach a statement to your return to show the income for the part of the year you are a nonresident. You can use Form 1040NR, U.S. Nonresident Alien Income Tax Return or Form 1040NR-EZ, U.S. Income Tax Return for Certain Nonresident Aliens With No Dependents as the statement, but be sure to write "Dual-Status Statement" across the top.

https://www.irs.gov/individuals/international-taxpayers/taxation-of-dual-status-aliens