The proper way to say this is: she speaks higher-status English than most native speakers.
I wonder if there's an implication that colloquial language is more complex than high status language.
The things being measured are different. To a first approximation, all native speakers do maximally well at sounding like a native speaker.
Lumifer's friend may indeed speak like a native speaker (though it's rare for people who learned as adults to do so), but she cannot be better at it than "most 'natives'".
What she can be better at than most natives is:
If it's worth saying, but not worth its own post (even in Discussion), then it goes here.