I accidently added someone as a real id friend in battle net, and they accepted. Now we both want to remove the person from the list, so that the other person shows up with the username instead. How do I do this ? I can't see anything on Blizzard's FAQ
There's not a way to "downgrade" someone's friend status.
The simplest solution would be to simply remove them as a friend, and then re-add them via Battle.tag alone.