Long-term experimentation shows that, at least as long as she's not getting bloodily injured in regular fights with Los Angeles's demon population (and she can generally make it through unscathed, and now every supernatural creature in this hemisphere knows that the Slayer can set you on fire with her mind and some demons keel over with no visible injuries if she just looks at them), she can operate in fine and suitably Slayery condition while being nibbled on most days. It would probably not be a wise exercise for Sherlock to attempt to subsist solely on her, though, so the brick house does acquire a fridge and a microwave thanks to Sherlock's slightly stolen inheritance.
This afternoon, Bella is sitting in an armchair in the house that is Jarvis, reading about some neutral demon species and taking dutiful notes.
[I can't see you, but are you keeping up well enough, can you cross its path - we'll see if it reacts, go from there?]
[Shit, are you okay -] Bella makes a jump for it and manages to land on its back. It doesn't seem to care. It doesn't slow down.
[I am less certain that engaging is wise,] he says, but he picks himself up off the ground and chases after them both.
She swings around its shoulders to try from another angle.
It grabs her off of it, hurls her to the ground, and steps on her in its unslowed quest for the school.
She doesn't move.
"Hello?" Charlie says to the phone.
"Bella has encountered some trouble. She is alive, and will almost certainly be fine, but she needs to be taken to the hospital. Please arrange for an ambulance at Chester and Oak; you can most likely prompt faster reaction than I."
He sets the handset of the landline down on the counter and radios for an ambulance, barking orders at the paramedics.
When it's on his way, he picks up the phone again. "Who are you and what happened to my daughter?"
"My name is Jarvis. She ran afoul of a Sunnydale hazard, which has since been neutralized."
"Sherlock is with her. He doesn't have a phone and hers may be broken, so he contacted me through other means."
Charlie hangs up and gets in his car to catch up to the ambulance.
Meanwhile, the ambulance is not far from the correct intersection. Paramedics hop out of it.
The paramedics load Bella into the ambulance and drive her away. They quite ignore the still white hulk of a demon, laid out on the sidewalk half a block away.
He wished to kill it; presumably it is dead. Sherlock approaches the demon and attempts to pick it up.
It is heavy. Heavy enough that without particularly trying to stomp, its step could leave a Slayer with all of Bella's manyfold blessings from her alt unconscious on the ground. Certainly too heavy to drag away. He can shift its hand or its foot, if he puts his back into it.