Kiriha had found Sanae in this hospital more than half a year ago.
After Koutarou and the others had defeated the hunters, Kiriha had given them a deal. In exchange for a large sum of money, she demanded that they would no longer attack Sanae and that they give her all the information they have regarding Sanae. The hunters agreed and accepted the deal.
The information she had received at this time included the fact that there was one more Sanae admitted to this hospital. Several years ago, she had been struck with an unknown disease and grown weak, and she had been forcibly admitted. In other words, Sanae wasn't a proper ghost. And the reason for why her parents hadn't returned to room 106 was because they had moved to their own parents’ house which was closer to the hospital.
Kiriha had doubted the information at first, but as her investigations continued, she became convinced that it was Sanae herself. After that, she devoted herself to curing Sanae.
There was only one single reason for Kiriha wanting to cure Sanae, and that was simple goodwill. Kiriha disliked fighting, and she was painfully aware of the feelings one felt when waiting for their loved ones. That's why she wanted to return Sanae to her parents as soon as possible.
Of course, this was a large minus for Kiriha who wanted to prolong the battle for room 106. If Sanae recovered and left room 106, the balance of power would shift and the radical faction would be stimulated. Kiriha would be unable to hold back the radicals as they would surely claim that this was their chance. So Kiriha began laying down the groundwork to cure Sanae early on, prepared that Sanae would actually secede from the battle.