Can my client safely say no to the heavy-hitting cytotoxic agent? Will she recover fully from her cancer without it?

We can give you an unequivocal yes to both. She does not truly need this heavy hitter and she will be healed and survive without it. But her fear is so great, and the doctors so entrenched in their thinking they need to pull out all the stops to get the greatest chance of success possible, it will be difficult to avoid this painful process. But, as we have shared with you, it will be survivable as well and, even though beside the point, the issue at hand is the outcome that she survives and returns to normal—that is the goal, and that is doable with divine support, and you are ensuring that she gets it.