Can dsomeone at the gym use a defibrillator on a woman with breast implants?

Yes. Done properly with an AED device, implants will not impede the ability to deliver defibrillation treatment.
Yes. Don't hesitate to use a defibrillator if it's necessary to save someone's life, breast implants or not.
Yes. Yes the right sided paddle will be above the implant and the left will be below the implant. It will not cause damage to the implant and the implant should not interfere with the defribillator.
Allow me to answer that question by asking another: Question by asking another: if someone was dying from asystole in your gym and their only chance of survival was to be defibrillated, why would you worry about their implants? I am assuming that the person in question is confident with respect to diagnosis and treatment of sudden death. If they probably shouldn't even be asking the question.