Your buddy is getting bad information.
Your warranty applies to your jeep as it left the factory. If you change something--like new coils or shocks--those new coils or shocks won't be covered. This makes sense of course, as why would Chrysler provide a warranty for another company's coils or shocks? The other company would cover them with their warranty, but Chrysler would not.
All the original equipment on your jeep will remain covered by Chrysler after you swap coils/shocks/whatever EXCEPT things damaged by the change you made. For example, if you put in 4" coil spacers, I guarantee you that the boot seal on your driveshaft would shred and need to be replaced. Chrysler would not be obligated to cover that under warranty (even though it's a Chrysler drive shaft) because the modification you made in fact caused the damage.
Almost nothing except odometer tampering outright "voids" your entire warranty. It's a piece-by-piece issue, not a total voidness issue.
So just remember: All original equipment on your jeep is covered unless you make a change that damages it.