You can do GPG by hand but if there’s one typo there’s a problem. Convince them to buy a smartphone if you want to communicate in an encrypted way; or write an app for the fliphone.

Which phone is it?

https://github.com/enigma-reloaded/enigma-reloaded/

best i’ve seen so far for a systematic way of copy pasting chats, decrypting and having them shown threaded.

secure encrypted plaintext messaging on top of another chat service probably doesn’t get easier than this. I would only advise to do the key exchange off piste on some other open channel so no one can say your random data sending has any intention they ascribe to it.