There are a few other things you and your friend can check:
- Make sure Real ID is enabled
- Blizzard Website > Account Settings > Security & Privacy > Privacy Options
- Ensure friend also has correct communication settings enabled
- Make sure the account is not muted by Parental Controls
- Make sure the account has not hit the friends list cap
- The cap is 200 friends
- If the cap is the reason for the unsuccessful attempt, it will not give any error messages. It will look like the request sends, disappears, and is not received
- Make sure the Blizzard account email is verified and is fully registered
- Make sure that the friend is not blocked
- Go to Blizzard App
- Click on the Blizzard symbol at the top, left-hand corner of the app
- Select the Settings button
- Click on Friends & Chat
- Click on View Block List
- If your friend is on the list, remove them
- Make sure both accounts are logged into same region of the Blizzard Application.