How can I get an external display that is mirrored to have a smaller resolution than what I see on my Macbook Pro?

I'm trying to help a sight impaired and somewhat physically impaired person. They have an assistant that uses the Macbook Pro while he watches on an external monitor.

The only thing is that for the sight impaired person to see what is going on while the assistant is working the resolution (mirrored) is so low, meaning bigger fonts, that it makes it difficult for the assistant to work on the Macbook Pro in that environment.

What can we do to remedy this? Is there any software to make a mirrored external display bigger than the computer?