According to Recruit A Friend Basics
While adventuring with your linked friend or family member, you will each gain triple experience while partied up to level 80, as long as your characters are within 4 levels of each other.
However I couldn't find enough information to answer the following questions:
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Is 'up to level 80' inclusive? In other word, do level 80 character get XP bonus?
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Do both characters have to be under level 80? If one character is level 81 and the other is level 79, do the level 79 character still get XP bonus?
Best Answer
In order:
Nope. Once you get reach level 80, your bonus experience cuts off, and you must earn the last few levels on your own, again.
You would continue to gain the bonus XP. Until you reach level 80, you continue to get bonus experience -- as long as you still have a partner within 4 levels, even if they are on the far side of 80.