I have worked at both Home Depot and Lowe's and both have the mirror mastic you are talking about. It is designed not to interfere with the silver on the mirror like most liquid nails type adhesives.
You say your mom is looking to put this on her counter? That raises a few questions.
First off is of course the safety issue. If you drop something on a mirror countertop I would think you might run the risk of having sharp shards of glass flying everywhere.
Secondly, mirrors scratch easily and like marble are not recommended for countertops because the metal pots and pans have a tendency to scratch the heck out of marble in a short period of time.
Thirdly, your mom might want to check the local building code to see if putting mirrors which aren't made with saftey plate glass are a violation of the code.