And how would Mexico’s government allow that to happen?
Entire cities, and even states, in Mexico are under de facto control of drug gangs, and the national government has made little or no effort to assert control. Assuming Texas has left the US and formed their own nation, given certain guarantees, I would not be surprised for the Mexican government to cede some degree of control in exchange for Texas to clean up the mess.
It would not have to be violent, or promise violence to those other than criminals. Instead it could be like a limited term peace treaty, with a part of Mexico acting like Hong Kong and Macau for 50 or 100 years.