Why demolish the floor? Repipes are done all the time by merely abandoning
old piping. Is there no attic?
Check you codes, it is probably unacceptable practice to put freshwater
piping on outside of the structure. If your sister lives in a cold climate,
furthermore, piping can freeze and burst. Insulation is recommended, even
for cold water pipes.
"Hank" wrote

The other option is jack hammering up her floor, since the
house is on a slab and the pipes are buried. I wondered if
the pipes could be run exposed on the outside of the walls
until they get to their respective fixtures. The pipes would
then be covered with some sort of trim.