tangential: I‘m using a oneplus 6 with postmarketOS but depending on your friend‘s it skills, it might not be ready for him yet.
So far its very usable but I suggest someone must want to swim against the current and do things differently. One could say a „pioneer“ type would be ideal for this.
Oh wow! Thats gnarly. On that note, have you looked at rustdesk? Tried it this week and it runs great. Its also open source.