For years I never turned in my FMM when I left Mexico by land due to ignorance of the proper procedure. As far as they knew, each time I had overstayed my FMM (even though I hadn't) but they always gave me a new FMM each time I entered without question. Your friend will be fine if he/she leaves by land. If they leave by air, then they'll have to pay a fine of a few pesos but they won't be denied reentry to the country in either case.
Gosh, if a quick trip to the border to get legal immigration status endangered a relationship, it probably was never meant to be in the first place!
Gosh, if a quick trip to the border to get legal immigration status endangered a relationship, it probably was never meant to be in the first place!