There is an addon for this athttps://addons.thunderbird.net/En-US/thunderbird/addon/read-aloud/ but reviews indicate it is still in formative stages. For your friend, I encourage using a more general tool so the individual can get text-to-speech on email and websites and documents. For example, here is one that states it handles email and documents and websites. Something such as this would provide a consistent tool for the individual. And, there are many others you can find on internet by a web search for text-to-speech software.

You might ask at https://groups.io/g/thunderbird which is a strong accessibility group.