I laughed at your “…she doesn’t want to install another app…”. I have a close friend that told me the same thing.
I was able to convince my sister to install Signal so that we could more securely send financial receipts back and forth, etc.
More than one of you in this thread have mentioned Matrix. I realized I signed up for it during the Great Migration about 3 weeks ago, but I haven’t spent any time there yet. How is it the same/different/better than whatever else? Curious. (I suppose Google might be my friend in this situation.)