since he is coming from a 2011 (15"?) non retina MBP, probably also a 13" rMBP would appear as a quite portable machine to him? I didn't read anywhere that it had to be the smallest laptop ever made?
and there are STILL (well, someday) the new rMBPs coming. I would definitely consider this option as well.
if you think so... fine. I have a different opinion. I think that the experience with ableton probably won't be very satisfying, if you use more than a few tracks (PS and LR are a different story, these aren't very demanding applications). and I also think the rMB is a crippled device.
roughly for the same price, roughly in the same form factor as a nearly perfect laptop (rMB: 1,31cm thick, rMBP: 1,8cm thick)
and yeah: I use hubs too, but let's face it: it just sucks, having to use them... they eventually add latency, etc.
... and finally this: I actually started on a ****** dual core notebook with ableton, I know what it's like (and I'm no professional either), it's the main reason I bought a quad core macbook - and I wouldn't go back to such an underpowered machine ever again (especially if I didn't had too). just look at the benchmarks man...
form is way > function here.