I can't really say if the xp is tied to GC rank, because i don't know that to be true. There are a couple of things i am sure about:
1. The xp you get from a FATE is directly related to how much you do in that FATE. For example,Gold contribution medals give more xp than silver.
2. Your current level will have a direct effect on your xp, compared to the "Recommended Level" of the FATE.
With that being said, is your brother under the recommended level? Is his friend equal to or higher?
His friend may have another class higher than his current class. Only real reason I know of. Which gives the lower class 50% more exp
I totally forgot about this. Edited, Sep 22nd 2013 4:27pm by Teneleven