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:
Is 'up to level 80' inclusive? In other word, do level 80 character get XP bonus?
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?