Confirmed,

I leveled a toon from 1 to 60 and was able to grant 30 levels.
I will level this toon further to see if he can grant more than that, but I'm 99% sure he won't be able. For one thing, the triple XP no longer applies once he hits 60. If the 60 toon is paired with 59 toon and kills a mob, the 60 toon gets normal rested XP, but 59 toon gets triple XP.