I don't have a good asynchronous solution, but if you can do it while on the phone/Teams with them, then Web Wormhole (https://webwormhole.io/) is absolutely fantastic. You both open a web page, you drag a file onto yours and it starts appearing in their download folder.
It's WebRTC based so peer-to-peer, E2E encrypted, with servers only used for serving the client, and for NAT punching.
What you really want is the native iOS or Android version of this (or of any Magic Wormhole derivative), which the wormhole-william people are working on (I think the Android version already works).
Yeah the Android version has been out for a few years[1][2]. There's an iOS target that works as well, I've just never published it to the app store. Every time I start that process I get annoyed at how hostile Apple is to open source developers and give up. Hopefully I can at least distribute a side loadable binary when that becomes a thing for EU users.
Apple's walled garden is supposed to be protected from trespassing, so software can't be required to trespass it as breaking protection is computer fraud.
Every time you hit the web page you have to re-trust that it's not stealing secrets from you. It's not end-to-end secure; in fact, you literally might as well just upload your whole file to that site and just let it handle the wormholing for you.
I tried wormhole.app many times and have about a 60% failure rate because there's an error or the download never completes. Don't know the difference with your link but i suppose i should try it next time
It's WebRTC based so peer-to-peer, E2E encrypted, with servers only used for serving the client, and for NAT punching.