This is kind of a high-risk, high-reward situation. Depending on what you write, you could easily make the reader notice you as a brave, unique individual who is differentiated from the pile of lame and repetitive essays on reporting a cheating friend. OTOH, you will need to provide a very good explanation of why you're not devious-- in other words, if you considered not reporting this incident due to its non-discoverability, can the adcom be sure you're not shifty in general and made the "right" decision on reporting this incident, but could just as easily have not made the "right" decision?
In general, it's better to write at least a few of your secondary essays with risky, novel arguments. Ideally, for at least one or two of them, your reader will click with your argument and give your candidacy a boost that the conservative essay doesn't get. Since med school is about getting one acceptance at a place you love, rather than getting "okay" assessments everywhere, it is much better to have five awesome secondaries and five sickening, reject him for writing this secondaries than ten conservative, no impact in either direction secondaries.