It's pretty much a chance fell into Hanako's lap. Aoi gets abducted by No. 6 and Yashiro wanted to save Aoi, Hanako knewing it would benefit Yashiro just went along with it. Plus it made Hanako's job easier since Aoi wanted to die in the first place.
DID AOI GOT SACRIFICED BY HANAKO TO SAVE YASHIRO'S LIFE SPAN?!?!?!?!?!?!?!!?!? OMG NO IM GONNA CRY（/TДT)/
It's pretty much a chance fell into Hanako's lap. Aoi gets abducted by No. 6 and Yashiro wanted to save Aoi, Hanako knewing it would benefit Yashiro just went along with it. Plus it made Hanako's job easier since Aoi wanted to die in the first place.