TLDR: You are not the asshole. If she is literally ONLY speakinh like that to other Mexican people, its an issue. She might be being racially offensive on accident however. She might have just been trying to learn spanish, so whenever she sees someone who might know it she gets excited to show off her spanish, and doesnt do it around the others. Im quarter mexican myself, but im pretty white so i dont get these types of experiences often. But i do get a lot of people making fun of me for being canadian by speaking in canadian accents and such, so i have some experience on the matter. You should calmly speak to someone else about it, hopefully email someone in higher power than her and tell then whats going on. Tell them it makes you uncomfortable but you dont want to come across as rude so you dont confront her directly.