In my opinion comparing the spec and weight only gets you so far, they are close enough that it really comes down to preference, in that regard the weight and size differences are negligible and it becomes a battery life vs. screen discussion.
I have had a MBA for two years and have been about to pull the trigger on a 13: rMPB for a while - primarily because I want more screen real estate. That is just my preference, I like a lot going on at once so more screen resolution is important to me.
Do I care about battery life? Certainly, lots of trans-con flights etc so in my case the hopefully perfect combination will be a Haswell rMPB.
I'd say if your brother mainly uses one app at a time in full screen then the MBA is the way to go as it technically meets his needs and you get amazing battery life. I he's the type that likes a lot going on (I think this goes beyond graphics work and includes coding, web design and mainly personal preference) then I think the greater real estate on the retina screen counts.
You can't really go wrong with either but for me a Haswell rMBP OR a Retina MBA would be the ideal, if and when...!