it may just have been because of how your friend's foot and your foot differed. Full tilts fit me like a glove, but when I first tried them on the pain was excruciating after about 10 mins.
Go to a boot fitter. There's a chance they fit, but there's a chance they don't. If they don't fit, there's not much you can do, it's pretty hard and time consuming to modify boot shells. Your flat feet may just be the problem though.
everyone who says "stop sucking full tilt dick" can blow it out their ass. They are pretty much the best boots on the market for our type of skiing, so it is certainly a bummer if you are stoked on how they work and then they don't fit. He was asking for advice.