Carl Schneider - Ciudad Mante, Tamaulipas, Mexico

My great-uncle Carl Schneider, an American citizen, moved to Mexico sometime in the 1960s.  He died in Ciudad Mante, Tamaulipas, Mexico on August 1, 1974. I have his death report from the Reports of Deaths of American Citizens Abroad database. However, I do not have his death certificate from Mexico; the U.S. Consulate in Mexico told me that Mexico will not mail death certificates out of the country.  My grandmother, his sister, went to Mexico when he died, and met a woman claiming to be his wife, but she thought it was some kind of scam. His death report said that the woman's name was Guadalupe Sanchez and that they were separated when he died. She was a young woman (my aunt said she was about 18), and my great-uncle was 69 when he died, and this woman collected his Social Security benefits after he died. I would like to know if they were ever really married. I would also be interested in learning more about my great-uncle's life in Mexico. I also know that he was buried in Ciudad Mante Cemetery.