I dont think they will get much support from the Mexicans.
I used to travel extensively in Mexico (and Central America) I was in a bar in Chihuahua Mexico a bunch of years ago. Some young (30's-40's) guys struck up a conversation. they invited me to a party. Turns out they were members of a Mexican weed smuggling operation...not Cartel, just a bunch of young guys, really bright and articulate, well educated. I asked them about that very question...the cartels helping to smuggle bad stuff into the US. Their opinion was that the Cartels see the US as a bread basket for their cocaine and know better than to do anything that would harm their business. Personally, they, the young guys at the party, for the most part like the freedom America offers the people. I cant say that that has not changed in the years since, seeing the brutality of what they do now, but I think the Cartels would see IS as competition and would do their best to eliminate that. Never saw any of them again. I hope they survived what has come to pass down there.