Miki just proved unintentionally what Rii is trying to say here. Without Yuki being the one they should take care for, they will be long dead in this depressing, no way out, zombie apocalypse. They were about to start a fight and rip apart their relationship but Yuki was there to be the one they should take care of. Sure its bad to rely and take advantage of her mental state but its better on both sides to do it in order to survive