I'm not a native speaker, but I have some US colleagues who write really poorly. I'm not saying this is the rule, nor that your friend was a bad writer, but the

I'm not a native speaker, but I have some US colleagues who write really poorly. I'm not saying this is the rule, nor that your friend was a bad writer, but the argument that a native speaker is de facto superior in the use of her own language is bogus. One of the very best pieces of English literature has been written by a Russian (Lolita by Nabokov) and there are many other examples. If a paper is written poorly I don't care if you're British: I'll point it out anyway.