my friends iphone 3g was stolen. she is in process of getting proof that she bought it, to go to the police and file a report. but what if the thief jailbreaked it, unlocked it, and is on a different carrier, not ATT,, how can the police track down or retrieve the phone?

The iPhone is lost. The police can't track it down like you're thinking. I think technically they can, but they wouldn't actually go through all the hassle just for a misdemeanor.

Tell her to file an insurance claim, if she has it.

how can they though

In theory you can track the phone by its IMEI number.

Although if your friends phone was at the latest release then you cant unlock it.