Weigh them and see what the diff is. Cut your good piston to match the other.
As far as that modification, it's something that Jennings talks about, it's to give the piston crown a little extra room to "grow" with the heat. Might want to closely mic both pistons, top, middle, bottom ,front to back, side to size.
In the end it's your decision.
You didn't mention that your buddy only had one good piston ,YO.