Just because your friend did it, doesn't make it safe. Most tire manufactors recommend a 8.5 - 10" rim for 12 1/2" wheels.
However, many jeepers will put them on 8" rims (the narrower rim creates more side wall bulge, which could prevent the rims from getting bashed offroad). If that bulge is too much though, it will make it easier for the tire to unseat (not a good thing on the highway). Many shops in my area wouldn't even mount 33 x 12 1/2 tires on a 7" rim.
95 yj, 4 cyl, auto trans, 4:88 gears, detroit front locker, rear detroit with super 35 kit, short shaft conversion, 4" superlift, 32" superswamper radials