It isn't sufficient to know that the student is cheating. You have to be able to prove it. Students, particularly the amoral, entitled students who are inclined to cheat, will cause an enormous amount of trouble for the professor/TA who accuses them of cheating without incontrovertible proof. The professor may dock the student's grade in some way, but you can't really fail them if it's clear the essay was written by someone much smarter or more fluent than they are.