To say the Axe sounds thin or lacks balls is like saying your car is too slow while refusing to put your foot on the gas. Or like saying your amp does not have enough gain without ever turning the gain up.
If he feels it's too thin, adjust the controls and make it thicker. It really is that simple.
I mean there are more adjustable settings on the Axe FX then any mere mortal could possibly require.
I think it is a case of someone needing to spend more time getting familiar with the Axe FX. Your buddy is probably just use to plugging in and playing.
When I run through my power amp and cab, I have to change the way I eq my patches that were setup for FRFR because they do sound thin.
However, by simply modifying the settings I can make it as thick as I want. I like to turn up the warm and thump settings in the amp sims. But also just eqing it to taste.
Too thin, not enough low end?.?. Well then, turn up the low end. There are about 20 different ways to do that on the axe fx, be it changing low pass filter settings, the graphic eq, parametric eq, amp controls etc...
This topic reminds me of why I always get a kick out of amp shootouts. So you get some guy who sets up two amps, plays them and seems to prove definitively one way or the the other which amp sounds better. You will have some guys come in and say, "yeah, amp A really lacks low end". Course if you look at the eq settings on the amp the guy may have the bass set next to nothing.