Good GODS, Aimee--what an asshat! I like your original email, but I think it can be dialed back. Want me to do a quick edit and paste a complete possible draft here?
The two main problems with chemo brain are short-term memory and shortened attention span. I don't know that you need to dumb things down, necessarily; you just need to make sure she takes notes and/or you also give her a written version of your instructions. She's just going to forget chunks of what you tell her, so don't mention something passing her in the hall and expect her to act on it. The attention span thing was harder for me. I was always easily distracted; after chemo, every five minutes it was "ooh, shiny." I think that things that will help make her focus, like short-term deadlines, might help. I'm thinking of something like, "I'd like 10 of these completed by noon" or "Could you please send me a status report by the end of the day?" Also, caffeine helps.
Holy crap. This is my brain ALL THE TIME. Luckily not quite as extreme, but I have ADHD friends who exactly fit that description. I didn't realize that chemo brain mimics ADHD, but wow. If caffeine helps, she might also look into ADHD drugs such as Ritalin, Concerta, etc. They can make a tremendous difference. I think this was mentioned at some point earlier, but I wanted to reitterate since it might really help. I know that frustration on both sides: I have to deal with my severely ADHD mom regularly, and she drives me batty. Then again, I know that *I* drive people batty for the exact same reasons, and the frustration I feel about not being able to remember or to focus is incredibly upsetting. Poor you and poor her. No fun at all.