"Non-Borrowing Spouse" is defined as the spouse, as determined by the law of the state in which the spouse and borrower reside or the state of celebration, of the HECM borrower at the time of closing and who is not a borrower.
Mortgagee Letter 2014-07
https://www.hud.gov/program_offices/administration/hudclips/letters/mortgagee
HECM Final Rule: Federal Register §206.3 https://www.govinfo.gov/content/pkg/FR-2017-01-19/pdf/2017-01044.pdf