If this person is still alive you will not find much on them after about 1940/50 or so (either on ancestry.com or familysearch.org). If they are very elderly and were living before then and moving around back and forth between the US and Mexico you may be able to pick them up in Border Crossing records (google familysearch.org border crossing records). You do need a general birth year to help identify them and some idea of which border they would be crossing (Nogales, Laredo, etc). Some of the immigration papers can be quite full of information. Again, if the person is alive you are crossing the line into personal and private information...and you will be restricted from access to this.