Thank-you both for your responses. I ended up sending a letter to the IRS, citing https://www.irs.gov/individuals/international-taxpayers/nonresident-aliens-dependents , bullet #3 which says:
"3. An individual claimed as a dependent must be a citizen, national, or resident of the United States, or a resident of Canada or Mexico."
And since she is a resident of Mexico, I asked the IRS to use my U.S. address in care of hers.
I'll let you know how that goes (in about a month). Thanks again for your help!
