A friend of mine has a 2013 GT500 and they use a seat cushion so his wife can see over the dash. Unfortunately, the red dye on the fabric cushion transferred to the white stripe on the leather seats.
You could try steam or Leather Master Dye Transfer Cleaner after you solve the defect my suggestion would be to recommend a seat cover so it can protect the seat when the seat cushion is used...good luck
Any kind of good leather cleaner should be your first attempt. Lexol would be a good one to try that you can pick up locally. If that doesn't work, then go for the LM Dye Transfer Remover.
Agree on the seat cover...or possibly a leather "coating"? Or even just a white towel between the pillow and seat so it can all be easily removed.