Can Rogers iPhone 3GS use Fido SIM card?

I currently have a iPhone 3G with Fido, it's great works fine. A friend of mine is getting an iphone for the first time and is cheap so wants to get the $99 3G, and she is on Rogers. Could I give her $100 to have her buy a 3GS instead, give her my current 3G and take her 3GS?

Just make sure the phone is unlocked. If not, there are a lot of free unlocking for iphones.
You'll have to change the APN settings, i.e. internet settings.
And congrats you now have the newest iphone

Rogers iPhone will not take Fido sim or vice Versa.

iPhone must be jailbroken and unlocked first. Also, you don't have to worry about changing APNs. The carrier info is built into the firmware of the phone.