I have a slip on one it's great, 2.5mm in thickness, with mine i don't have to pull the lens out. If your mate made these for a £5er each or something like that i'd buy without question.
no they have a sleeve that they sit in that goes over the original lens, so if that gets shattered i can replace it in the field.
since i've had mine though (its been hit numerous times and still not shattered) i haven't had to replace it so i have a few just sitting around in case. i think if i was that bothered about getting the contour to look as it should and it was cost effective id probably buy off your friend as well, but their is always the option of the tried and tested airsoft optician.