Activity sharing not working - sharing with old email

Wondering if anyone has come across a similar problem. I am having trouble sharing with a single person. Whenever we send share requests their name appears in my sharing list for about 10 seconds and then disappears. My activity will however continue to be shared to them. When we initially shared information years ago we had trouble sharing, and somehow got it working by sharing from their school email instead of their Apple ID (not sure I remember how we managed it). Since they do not have access to this email anymore they removed all traces of it from their Apple ID, but activity continues to share from this email. If they send me a competition request I see it is being sent from their old email still. It seems like our sharing is still linked to their old email, despite removing friends from both sides.

I have tried all of the usual suggestions: repairing watches, signing out of iCloud, remaking contacts. Nothing works.