Granted each border crossing seems to set their own rules but we have been told by INM at the NM crossing that the former FMT/FMM were multiple entry/exit as long as it was within the 180 day period. We took their advice once and forgot to pay attention to the 180 days but that is a different story.
All I know is what it states on the FMM, an official Mexican document on the back just under the word Important, the first paragraph says=
Durning your stay in Mexico, you must retain this immigration form and surrender it upon leaving the country.
That what is says in black and white, not word of mouth.
It does not say a border guard will make up his or her own rules, a border guard could tell you anything and next time through there will be a different one or you have 30 days left, visit Guatemala for a week or two and save 295 pesos when you re-enter Mexico, I am sure the husband of the cousin told her to try and get away with re-using the FMM and not this lady from France ....suerte