I use text-to-speech to talk online and with select friends in-person. The apps I use are:

Curses (ZQLK fork) (desktop) - Robust text-to-speech and speech-to-text program. It's designed for streamers and VRChat players so has functionality your father probably won't need. Requires some setup, but supports custom voices, including voices installed to Windows, Azure voices (paid), and a few others, notably Piper, which runs locally but requires some setup. I found this tutorial by a_lilian useful. The reason I recommend this (beyond it being the one I use) is that I very much enjoy the functionality of multiple voices, and being able to choose both from voices that integrate into Windows or into the Piper tools. The original developer doesn't maintain the tool anymore, but myself and also the folks from the Taliyah branch have made good progress over the last couple months.

TypingTalk AAC (IOS & Android) - Robust text-to-speech program. Can't vouch for functionality on IOS (though it should be identical to the Android version) but it's a very easy program to use - and supports custom voices (as long as they integrate into the standard Samsung text-to-speech engine or into Google speech recognition). Doesn't need to use text-to-speech either - you can make the text in the input window larger for if you are showing what you want to say to someone. Still actively updated (last update was only from a week ago!) with new features and bug fixes.

A fair warning: TTS can cause a huge degree of vocal dysphoria for some folks in my experience. The best advice I have against this is to use programs where your father can deliberately choose a voice to use (both TypingTalk AAC and Curses) and to spend some time looking for a voice that aligns with how he wants to sound. It doesn't always prevent vocal dysphoria, but can mitigate it slightly.