It seems the answer is Brian Warner's magic-wormhole. You're gonna see lots of file transfer sites with wormhole in their name, but if you want security you should use the original one, which is BW's m-w.
It is implemented in Python [1], so it's hard to install.
So someone made a Go version of it [2] that has binaries for windows, Mac, Linux, BSD etc. But it's command line so maybe not suitable for lay people.
So another person made a GUI for it that also has binaries for all OS [3].
Also there is an android app [4]. Someone needs to implement an iOS one.
[1] https://github.com/magic-wormhole/magic-wormhole/
[2] https://github.com/psanford/wormhole-william/
[3] https://github.com/Jacalz/wormhole-gui/
[4] https://github.com/psanford/wormhole-william-mobile/
TLDR: ask them to install [5] and [6].
[5] https://github.com/Jacalz/wormhole-gui/releases/
(click on 'Assets' under 'Latest release' and download the zip or tar.gz for your OS)
[6] https://play.google.com/store/apps/details?id=io.sanford.wor...
Try it, it's usage is cute and really feels like magic.
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Edit: Discussion from few days ago. The creator is in the comments.