I have a friend who wants to be introduced to RPGs. We played a campaign in a game system of my own creation that went pretty well, but I want to show them a variety of systems so that they gain more familiarity with RPGs in general, and also to bring in other people so that they get more of an idea regarding how RPGs typically go (i.e. not one-on-one campaigns). To this end, I have some people and we are gonna play Pathfinder for our 'generic D&D stuff' game. Unfortunately, this friend is autistic and becomes nonverbal for extended periods of time (6 hours to several days) when exposed to intense negative stimuli. They can still communicate via typing, writing, and mewling most of the time, but they can't talk. They also don't know very much sign language, so we'd prefer not to rely on that (we're considering using this anyways, as it would be a good method to motivate language learning, but we'd prefer a better option).
They have been nonverbal for several months now, on account of an ongoing campaign of systematic discrimination against a minority they are part of on the part of the county in which they live and their resultent paucity of resources. Despite state opposition to the campaign, given the current federal political situation, it seems unlikely any legal progress will be made in the next several years, so I'd prefer to not just wait until they are able to talk again, which had been our plan previously.
While running online games is a possibility, that medium fundamentally changes the experience of an RPG and I want to run in-person games in addition, which means we need some way to engage in in-person RPGs where one member is unable to talk. How can I enable them to participate in the game without being able to speak?
Your answer should explain how the techniques/tools/etc you propose have worked in practice for you or someone else and should be reasonably expected to be at least as effective as ASL pidgin, at a minimum.