In this specific instance, the tier rules trump the "play play play" rule. Allowing a PC to play up two tiers and then continuing to allow him to do so for the life of his character, would cause that PC to be unfairly well above other PCs of his level with regard to wealth and gear. You can play up one tier if it's the only way to make a legal table but playing up two or more tiers is never allowed.
If he's really that far behind the other players, the other players should consider making new level 1s and leveling up with their friend instead.
If he's already got a level-appropriate character with the group, then he should play that character.