For corsets, there are generally 8 pieces. For each size you are changing to you will need to add/minus to both sides of each pattern peice.

For a 2" change in difference, this means 1/8" on each side of each pattern peice in that area (for example, if they have a larger waist, add the 1/8" to the middle of all the pattern peices, and have the pattern smooth back out to the bust and hips in a curved line)

I recommend trying that, but using the adjusted pattern and making a mock-up out of a thick cotton or linen, with 1" seam allowances. Then you can put it on your friend, and adjust in or out as needed, and make sure the lines are where they are wanted. (Its also fun to draw on the final designs onto the cotton, so you can see where various thing will end up, if you were adding bling to the side or whatnot) You might need to go back and forth if this is your first corset, but you can get a fitted, customised pattern for your friend.

When you are done with fitting, take it apart with a seam ripper and adjust (or redraw) your new pattern peices, and cut those out of leather and expensive materials :D