Q: My cousin jailbroke her iPhone 4 (still under warranty) and realized it was stupid and immediately restored. If she breaks the phone, will Sprint or Apple replace it?

No. Jailbreaking voids the warranty and any AppleCare agreement entirely.

It is not always possible to return a jailbroken phone to a pristine state.

The phone is now unsupportable by Apple or on these forums.

Before you jailbreak a phone you are informed of the disadvantages and what you VOID prior to performing the procedure. She is responsible for her own actions. Welcome to the real world

Incorrect. Restoring to factory settings on iTunes erases the jailbreak and it is good as new.

Not necessarily. Depening on the method/program used. Many users have posted here of errors and not being able to restore after jailbreaking.

Sorry Ladasha, but you're wrong. If the baseband is modified, it's impossible to revert to normal.

if you restore stock firmware, the baseband is updated. you guys really dont know what youre talkin bout do ya? unless the iphone 3g/3gs has ipad baseband installed, a restore to stock firmware will work. stop being such  goody goodys, to the OP, do a DFU restore to the latest firmware