If you are desperate, you might have to just trust that person your login/password, ask them to login to your acc and change/add phone number.
If not, another one I was thinking was asking them to meet at their local police department and looping in an officer into the story or something. Different things work for different folks so they'd have to get creative to find the right solution.
Oh wait, I believe these are called notary offices in meatspace!
(tangent: at some point I thought that maybe notaries could sign one's PGP key instead of relying on rare/non-existent signing parties)