The Garmin devices have (or at least had, not using mine anymore) an option to share Geocaches wireless between them.
I suggest implementing something similar. It could be done in two ways, either local or online. I don't really know what's built into Android for this, but I figure that one should be able to create some form of data packets and then use the NFC technology to transfer it locally by letting the phones touch each other.
For online it would require some (free) service to send smaller packets in some way. I have not researched if this exists. Or if encoded messages could be sent via chat services like Messenger/Telegram/Whatsapp and similar. I honestly don't know.
I actually expect this to be more work than use, it's not something I miss often myself, but still, a useful idea.
Customer support service by UserEcho