I'm no whiz when it comes to stuff like this but from I've been reading the last little while there's a relationship between water hardness and water alkalinity. The below article explains it nicely. I bolded the part that interests me the most.
I'm thinking if her water is that hard then her water is very alkaline as well unless it's treated before it gets to the toilet.
ALKALINITY VS WATER HARDNESS
Alkalinity and water hardness are fairly similar - essentially they both come from sources in nature. Water moves through rocks (and picks up minerals as it does so) on its way to rivers and lakes. When limestone and dolomite dissolve in water, one half of the molecule is calcium or magnesium (the "hardness") and the other half is the carbonate (the "alkalinity"). This means that the level of water hardness and alkalinity in a place will be very similar. However, they are very separate measurements, and have very different importance.