A handheld tuner, in my case a SCT SF3, get's locked/married to a car's VIN. When you connect an unlocked handheld to your car the 1st thing it wants you to do is download the stock file/tune to it. This locks the handheld to that car.
Now the actual tuner/person that writes the custom tune for your car can download to the handheld and then upload it from the handheld to the car.
To unlock the handheld from the car it's married to so you can use it on a different car the 1st car HAS TO BE RETURNED TO STOCK.
Someone would have to return their car to stock tune for you to use it. Then you would have to return your car to stock tune to give it back. There's nothing accomplished and you used 2 of the 5 unlocks on the handheld.
In short, you pretty much need your own handheld.