I've got a friend who knows about Army guns and knows how to shoot, but is rather ignorant when it comes to gun ownership on the civilian side. I've been trying to educate him on the legal aspects, cost aspects, all the stuff you don't have to concern yourself with going to the range at work. He's been to the range with me numerous times on the civilian side and has paid for ammo every time (and by that I mean he went to the store and bought it). When we went to an indoor range he paid the range fee since we were shooting my guns. I think he'll eventually buy his own rifle; I keep telling him he should buy a FN SCAR-L. He came out to the range with me just a week ago and some friend of his came along who hadn't shot in over 4 years. They brought ammo again, so no complaints.
I don't insist on someone paying for ammo the first time they ever come to the range with me though, especially if they just shoot 22LR. The wife has taken a mild interest in guns and tolerates me having them as a hobby. Her mom and a friend of hers have both come to the range with us in the just over a year we've been gun owners. I didn't make them pay for anything.
I wouldn't continue to take "friends" to the range though if they don't pay for ammo. Maybe if the only thing they shoot is 22LR, maybe. Any centerfire though, they need to BYOA. I'd suggest taking them to a store with you to pick up targets and ammo on the way to the range so they get a feel for how much this stuff costs. After that, if they don't start buying their own centerfire or at least buying you beer or gas or something ...
I wouldn't cease being friends with them, but I wouldn't continue to let them shoot for free. If they want to continue going to the range for free, they should join the Army.
Oh, that friend someone mentioned who refuses to buy ammo because she doesn't want to give those evil gun companies money ... ummm, she does realize the U.S. Military buys from those same companies? I.E. her tax dollars are already supporting them. Not all of them, mind you, but FN, Colt, Winchester, H&K, Beretta, and Remington just to name a few.