Yuki's problem isn't particularly difficult for Fang Zheng to solve. You could even say that as long as they return to the real world, Fang Zheng has plenty of ways to handle it.
The Swarm's genetic adjustments can definitely cure Yuki's problem, and if that doesn't work, Fang Zheng can use a Spirit Crystal to transform Yuki into an Elf. He still has three Spirit Crystals in hand that haven't been used yet. If he gives Yuki one of them and transforms her into an Elf like Kurumi Tohka, then there won't be any issues.
For now, Fang Zheng only hopes that Yuki can hold on, because according to Yuki's statement, the reason she entered this game initially was that the doctors used it as part of "end-of-life care treatment" as an experiment.
In fact, long before the game started, Yuki had already been living in a sterile room, leading a day-to-day existence.
