Applescript to change outgoing mail from html to plain text

I have my Outlook set to send messages in html. I want to send messages to one person in particular in plain text. I would like not tohave to remember to switch those messages every time I hit command-r or command-n. I do not want to turn off the "reply in the sender's format" option -- this is behaviour I want for one recipient only, not all my mail.

I can make a rule for outgoing mail with that recipient's email address -- and I could assign an Applescript to that message.

Does anyone know how to write an Applescript that switches an outgoing message from html to plain text?

Or does anyone have another idea?

(To explain why I want to do this, I correspond with someone who uses Mac Mail and has poor eyesight. He has set Mac Mail to enlarge all his mail to 25 point size, but Outlook's html settings defeat that. If I send in plain text, his preferences work.)

Thanks,

Madeline