USC with with spouse willfully abandoning GC

I’m a US citizen with a GC spouse. We have moved to a foreign country and my spouse is planning on giving up her GC this year. She is not a long-term resident. We will be filing MFJ this year, but next year we will be filing MFS. She has no US sourced income. 

Next year, when I file MFS, for “Spouse’s social security number” should I continue to use her existing SSN, or should I enter NRA?