This is a strange one...

1. Taxpayer is a US citizen lives in the US

2.  Spouse lives in Mexico. They were married in Mexico and husband supports wife and kids in Mexico.

3. Non of his family have SS or TIN number

I am on boarding the customer and I saw that he filed single last year. Based on my research, he should have filed married filing Jointly but he wants to file Single as in previous years. I did not prepare previous tax returns. Thoughts?