I wouldn't discount the value of having some rudimentary signs to communicate immediately after surgery. It seems odd to me to focus on some dream of a perfect TTS synthesis if these more basic needs are not addressed first.

If you've ever had a mouth injury that inhibits talking, or been in a foreign environment where your speech is totally useless, it can be very stressful to be unable to communicate. I think the couple should consider learning some of the basics ahead of time, so that communication is possible without typing or any other apparatus.

Considering post-surgery recovery window, I'd want to be able to express very basic things like:

I am comfortable

I am in pain

I am hungry

I am nauseated

I need to urinate/defecate

I want to rest

I love you

When will you return

etc. I might suggest trying to boil down one or two inside-joke kinds of phrases as well, to be able to lift each others spirits in private or intimate way.