Eventually you will probably cut through the boot but, unless you are in a cheap recreational boot, I'd imagine you will need new boots due to the plastic getting brittle and cracking before you have problems with cutting through the boot. I'm 5 years in to my current Head RS130s (each year I have skied 30+ days) without any problems. And yes, the insides are quite cut up. Actually, what I have had more problems with is cutting up the insides of my ski pants and ruining them!
As to rental equipment, that is not an easy question. Rental boots generally suck. You just have to go with what fits the best, unfortunately. They are made to almost fit everyone and actually fit no one.
Without specific models of skis, you won't get much of an answer. Too much variation from model to model. Best bet is probably going to be looking for something that is <70mm underfoot (72mm in a pinch) and, if your brother is of average height, a length in the 160-165 range. They won't be ideal, but its likely the best you are going to get.