Good morning, @TNews ...
Hmmm...unfortunately, I was unable to replicate this. I navigated to my own Canvas Inbox to make sure things were working. For the most part, I used the Tab key on my keyboard to get to the icons I needed. Then, when I got to the area to compose a new message in the Canvas Inbox, I mostly used the down/right arrow and the Enter key to select the name of the course, and then I was able to select the option to message all instructors or to select them individually from the course. So, everything was working properly on my end.
Does your student have extra software installed (such as a screen reader) that might not be working properly? Any other browser extensions that might be interfering? You might need to reach out to Canvas Support so that they can dig in to the issue with you and the student a bit more. How do I contact Canvas Support?
Keep us posted here in the Community...thanks!