That's fear mongering. 120 million Mexicans versus 10 million Americans with Mexican ancestry maybe? Doesn't seem likely that there will be more Mexicans in the United States than in Mexico in a few years. Many of them are going back to Mexico, and many others move from the countryside to the cities within Mexico instead of going to the United States. That's the trend reversal I mentioned before. The United States aren't the attractive destination they once were anymore.
As per the 2010 Census of the United Sates, there are a total of 50,477,594 Hispanics residing legally in the United States. Add in the 13 million or so illegal immigrants, the figure will cross 20% of the total population. This is much higher than the 10 million figure which you had quoted.
Also, there is no strong evidence to prove that more Mexicans are going back to their home country, than those who migrate to the United States. The immigration influx from Latin America seems to be growing day by day.
You're right, Americans with Mexican ancestry are more than 10 million. About 35 million legally, and maybe the 13 million you mentioned illegally. But do you have anything to back up the idea that emigration from Mexico is increasing? The Pew research seems to contradict it. Has anything dramatically changed since the study, or anything that shows it wrong? Especially in the scale needed for there to be more Mexicans in the United States than in Mexico in a few years like you were pushing?
http://www.pewresearch.org/daily-number/net-migration-from-mexico-falls-to-zero-and-perhaps-less 
