Yeah, its a red flag.
They indicate potential problems, which is why you look for multiple, but even one obvious one is enough to make you a bit cautious, it makes sense.

But if its someone I know, and who isn't a native English speaker and is pretty chill, I'm probably going to let it slide for a couple uses, then gently ask them why they're using a certain term in a nice pause in the conversation.

I'm lucky to have had friends from lots of other places, but it can make things so awkward with corrections, even if it has to be done.
But people can absolutely wind up learning horrendously out of date language without meaning any malice. I've seen it happen because of confusion regarding the existence of the NAACP, and the use of the term, "People of Color."

The way OP is talking makes it sound like he could likely talk it out with the other people, and I think that may be the best option. (though I can understand why they may want some insight from the internet first)