Use special font for one recipient

Outlook and Outlook on the Web has it's own accessability options and accessability checker however it's metrics are owned by Microsoft and you cannot configure them on a tenant basis at the moment, which is a shame.

You can see the detail here: https://support.office.com/en-us/article/Make-your-Outlook-email-accessible-71ce71f4-7b15-4b7a-a2e3-cf91721bbacb

I've appended two pictures to this message so you can see where it's enabled and how it looks for end users. You'll also notice you can enable 'light version' under accessability settings to help here too.

In regards to the conditional formatting - this is probably your best bet but I would enable accessability checker for your visually impaired user to remind other users in your Org when sending to them - seems easier than making a policy tip.

Here's a working link for conditional formatting: https://support.office.com/en-us/article/Automatically-change-incoming-message-colors-and-fonts-based-on-sender-subject-or-recipients-4EFBF993-FB00-4F2C-9A3F-78E64E4455EC