They do not soften the water. The claim is that they separate the minerals such as calcium and magnesium as tey flow through the pipe by the magnets. It does not get rid of anything although the magnet salesman make lots of claims about them.
You need a real water softner to do an ion exchange to actually remove those minerals to soften the water.
It's your parents money, they can waste it if they want to. Ask if the company will install it and let you try it for 3 months and if you like it you pay and if not then they remove it and see if you have any takers(get it in writing). No matter what you do, you need to start with a water test and check with the neighbors and other in town to see what they do. Talk to a softner company also.