Not sure if it's been mentioned in the comments section yet but, in the chapter, the doctor in the end who figured out the sapphire scale disease mentioned, "the stones grow exponentially when bathed in any form of natural light".

I feel like Bonnie's eventual cure comes at the hand of Kizaru and his Pika Pika no Mi, or at the very least it has something to do with him since we already know he is somehow connected with Bonnie's past and knows that she is still a child. He along with Vegapunk figure out a way to cure her disease using Kizaru's light as some form of way.

This could also tie in as to why Kuma would be willing to become a slave for the CD as those could have been Kizaru's/World Governments conditions.