Mailed item not received
Hope is the carrot dangled before the draft horse that plods along in the vain attempt to reach it
Try having your friend delete some of the mail in his mailbox. This usually happens if someone has reached the limit of 10/10 or 7/10 as there is an issue with the number of mails you can hold.
Guild Leader for [Myth] Darkhaven
The limit of 10 messages only applies to mails sent by other players (envelope icon); the mails sent by NPCs (e.g. renown heart completion messages) do not count.