This is most major chain restaurants. There certainly exist some real lazy spics, but I’ve never seen a lazy spic line cook. It’s the perfect job for them. It’s fast-paced, and the only mental acuity required is timing. 9 times out of ten, if you receive cold food at a restaurant, it’s because the wait staff was dragging their ass.
As far as knowing Spanish goes, your friend only needs to know affirmative words, the names of some items (they’ll know the menu items, I’m talking about eggs, etc.), and the names of the cooks themselves. Really easy. Even if your friend came in new and that was his very first line job, he’d be looked up to as as leader because he’d likely be the one directly interfacing with the back of the house manager, regardless of tenure.
A lot of those Mexicans speak English fairly well too, they would just rather fly under the radar knowing that if word gets out that they understand/speak English well, they’re going to be the “go-to” guy for stuff.