Hi - My Mother's Heat Operates At 20% Capacity And

Hi - my mother's heat operates at 20% capacity and she has had a pacemaker for 5 years now. Recently she underwent a double amputation of her legs just above the knee which she has recovered from very well. She now wants to go skydiving and her GP is reluctant to write a letter saying she will be OK to do this as he thinks it could cause a heart attach as the place will be unpressurised and she will be going up to 10,000 feet. Was looking at getting a second opinion - any advice?