You must complete all friendship quests for that character in order to receive the level 10 reward. You will know when you are done with the Friendship Quest for that specific character when you no longer have Friendship Quests listed under Quests and the character in dialogue gives you your reward! If the character is not giving you their final quest, check your Quest Log to make sure there aren't any prerequisites that need to be done first.
Note about Rewards: There are some level 10 rewards that may not be in the game just yet, so you may complete all the Friendship quests and not receive your items. As of right now, Wall-E and Minnie are Characters that do not seem to give the reward after doing all quests at this time. We cannot confirm or deny if this is intentional or a bug at this time.
**Attached is what a final quest for a character would look like, this one being for Merlin! **