Years ago, some friends and coworkers and I all had a pretty open discussion about it when I was working in construction. It was very multi ethnic.
Basically, it depends on where you are and who the predominant Spanish speakers are and have been. It's just stereotyping, ignorance, and confirmation bias. We realized that for a long time, on the West Coast, a pretty big majority of people that were Hispanic were of Mexican heritage, but the more east in the US you went, it stopped being the case. Mexico is West, the Caribbean is more east, go figure.
Now, if somebody has faced being wrong before and doesn't learn or doubles down or is willfully ignorant... That IS full-blown hateful racism.
But, truly, a lot of people have just honestly never been exposed to [brown-skinned Spanish-speaking person not being Mexican], and until they do, it's not a low percentage chance to assume that's the case.
The power of seeing things as default is strong, and we should not assume certain purposes or facts. It's the same way that language, at least in English, we no longer default to he/him when talking about a hypothetical person. Not just because it's ethically or culturally wrong, like a lot of people think, but because it's also factually wrong.
Another thing to consider is that just because an assumption like this is made, doesn't mean it's hostile. There are definitely varying degrees, some more insidious than others, and it's pretty easy to get lost in the weeds and go down rabbit holes of philosophy here (important, but just not here, not now)... Another time when I was on a construction site, some Mexican guy walked up to me and started politely asking me questions in Spanish. I can't speak a ton of Spanish and I'm not Hispanic at all. I told him that, and we ended up having a funny conversation about it and he was really surprised that I not only wasn't Mexican, but not Hispanic at all.
Assumptions are everywhere, and assuming somebody is a bad person because they entered a country illegally is a really dumb assumption. People are people, and have tons of good and bad reasons for everything. Philosophy, knowledge, fear, values, trauma, privilege, and perspective ALL form us and our assumptions and actions. Maybe that person is from Mexican, maybe they're from Cuba, maybe they're from Canadia, maybe they're a hundred-generation American and can trace heritage back two thousand years and don't play by your silly rules.
We're all varying degrees of racist. We can't deny that.
-But!
We have to do our best to check ourselves, improve ourselves, and try to be better people. We're going to make mistakes! That's part of learning and we have to learn when to give ourselves and others grace.... But also when to condemn actions, words, and perspectives.
White conservatives are often super hateful, ignorant, fearful people, running on overdeveloped amygdalas and underdeveloped social skills and empathy. Not always, but a lot of the time. Faith and taboo lead design a very specific worldview and "understanding" of reality and expectations.
Pity your friend's weird dad, but do have caution of his ignorance and hate. Know that he may be intelligent enough to grow as a human and change, and if you feel the need to spare the risk and resources, perhaps you can make an attempt to educate him into the 21st century.
TL;DR: Because humans suck and none of us are as smart as we think.