How can I convert an AT&T iPhone 3GS to a Verizon iPhone 3GS?
You can't convert a GSM iPhone to run on the CDMA network, unfortunately. Neither the functional nor the cosmetic components are interchangeable.

If your friend is willing to give you his iPhone and if it's still in decent condition you could probably sell it for enough to fund the purchase of a Verizon iPhone. If it's not in great condition and you still have the urge to tinker, you may be able to find cheap cosmetic parts to restore it to salable condition.