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I've read all the various responses so far, and I haven't seen the most obvious one. Act as if the CEO actually does understand the situation. Say to the CEO (or email to the CEO if you're not on speaking terms) the following: "Just to clarify, you want me to devote a substantial portion of my time over the next three months to mentoring the NMG as he gets up to speed on our product and on technical writing as a craft, right?" Either the CEO will confirm that that is the explicit plan or the CEO will realize that the situation is more complicated than it first appeared. Either way, you've given notice that your time will be impacted and that NMG has a lot o' learnin' to do.