PHOENIX — Suny Santana hadn’t been working long as a prep cook at St. Francis, a stylish restaurant in the heart of this city, when his employer learned that he was an undocumented immigrant, brought by his parents from Mexico to the United States as a 12-year-old.

Timing and luck were on Mr. Santana’s side: The boss, Aaron Chamberlin, told him he could keep the job if he found a way to legalize his status. Months later, in 2012, the Obama administration began the program known as Deferred Action for Childhood Arrivals, or DACA, giving young immigrants like Mr. Santana temporary permission to live and work in the United States.

Mr. Santana immediately applied, and thrived. His skills, hunger to learn and determination so impressed Mr. Chamberlin that the restaurateur offered to put him in charge of a restaurant. In November, the two men plan to open Taco Chelo, a modern taqueria where Mr. Santana, at age 24, would be a partner and the chef de cuisine.

This time, though, the timing is not so good.

Last week, the Trump administration announced that it would end the immigration program in six months unless Congress takes action. Mr. Chamberlin says the move will not derail the opening of Taco Chelo, but Mr. Santana still worries about the future of their partnership and the life he has built in America.