When I was wheeling 5-6 times a year on relatively mild midwest stuff I'd clap out a set of YJ springs by the end of the season. Unfortunately I haven't gotten out in the past couple years like I used to.
I don't see how your buddy's YJ springs keep their arch, unless they aren't stock springs, or he's running so much bump stop they can't invert on compression.
Maintaining them by swapping spare main leafs and/or main leafs w/ eyes cut off was normal "yearly maintenance" on my YJ, esp. on the fronts where I didn't have the traction bar limiting flex a bit. Its worth it though - they flex really well for the price, which is usually pennies on the dollar if you can get your hands on used ones fairly easily. I'd never try to talk anyone out of using them but they aren't an "install it and forget it" type of item either, in my experience.
They do seem to go negative to a small point and then seem to stop getting worse though. My fronts are like that and have been for a couple of years because I haven't repacked them. I think they almost ride better that way though, and I'm not wheeling and still have enough caster, so it doesn't really matter.