Question about jailbroken tethering

My friend believes his jailbroken tethering app doesn't deplete his data plan. Is this correct or is he full of it? Thanks in advance

He is completely full of it. Of course he will find this out himself the hard way when he exceeds his limits. TTNSTAAFL.

Your friend is correct in that if you don't use the the built-in tethering and use a third party hack that provides the conduit outside of the OS, you will not need to incur a tethering fee as you would on most US providers.

With that said, as chas_m mentioned, you WILL use data and if you exceed your cap, you'll pay for it just as you would in any other circumstance. Additionally, the US carriers are currently on a mission to track down people who use unauthorized tethering to force them to pay for it. Tracking it is child's play, since all they need to do is determine what MAC (address, not computer) traffic is destined for or originating from and compare it to that of the phone.

If you are found to be tethering unauthorized, AT&T will automatically put you on a tethering plan. With Verizon, I've heard stories that they will find you in violation of your contract and penalize you as per its terms.

Either way, it's not something I'd play around with unless you're ready for battle.