Memorizing grammar rules and technical vocabulary is not particularly difficult. I do believe nonnative speakers are better at textbook grammar than natives. That, however, doesn't mean they can pass for a native speaker in writing or speech. Get someone who knows all the ins and outs of English grammar, but never been to an English speaking country and have them write an essay. Just about every native speaker will be able to tell the text is not written by a native speaker. Grammar is trivial, but language is hard.