Softening the seat leather

Well, I’ll try and summarize the story.

My Dad took a 12 pack of Diet Pepsi to his work on a day that was over 110 degrees Fahrenheit and forgot it in his car when he went in to work. I think you can see where this is going. All of the cans exploded and rocketed around the front seats. Big mess!

The problem my Dad has now is that the passenger seat leather is so stiff it feels like sitting on a plywood board.

I have tried leather softener and an aerospace protector called 303 to no avail. Is there anything else I can do to try and soften the leather?