Check:
- pending friends request, if yes cancel them in the battlenet app
- already on blocked list, if yes, delete there
Then ask the other player to do the same.
Exit game (and app if running).
If it still does not work create a support ticket.
There had been times when there were issues with the friends list. But currently you are the only one reporting it, so I assume it is an account issue, either yours or the other players.