You mentioned this is about your son-in-law---so assuming his spouse is your daughter----some information for your daughter:
MY SPOUSE LIVES ABROAD AND/OR DOES NOT HAVE A SSN
https://ttlc.intuit.com/turbotax-support/en-us/help-article/taxation/married-person-claim-head-house...
https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-credits-deductions/file-taxes-spouse...
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
