I ran into this problem too. Thing was, it was the same recommender who had offered to write me one. Oy.
Honestly, I don't see a huge ethical problem in writing/outlining a recommendation and then your boss editing it before signing and sending it on. It's not the way I'd prefer to do it, of course. And if you have ready alternatives, it might be better to go with them. But if you don't... eh. I think the ability of adcoms to recognize your "writing style" is overestimated. They do read thousands of essays, of course, and if your writing style is sufficiently sophisticated (i.e. doesn't include obvious errors of grammar, spelling and awkward sentence structure), I doubt they'll notice.
Still, when I'm a big important person who gets recommendation requests eventually, I'll make it a point never to ask a subordinate to write their own. I just think it's tacky.