As Doc stated, your son-in-law is wrong, wrong, wrong.
Using the e-brake is harmful?
On the contrary, failure to use it can be harmful to the car (by placing more strain on the transmission’s “park pawl”), and potentially harmful to people and other cars when the car rolls away (through failure of the “park pawl”, which is rare, but does happen).
In fact, using the hand brake on a regular basis is the only way to be sure that it will actually function when you might need it in an emergency. An unused e-brake will tend to develop rust on the cable that activates it. The cable and related linkage can rust in place, thus making the e-brake inoperable.
As Doc stated, an e-brake can freeze under the right (or is it wrong?) conditions, so you might want to avoid using it in wet conditions that will be followed by falling temperatures. Then again, the e-brakes that tend to freeze under these conditions are the ones that are rarely used…
Hopefully, your son-in-law is knowledgeable on other topics, because he has no clue regarding this automotive issue. Our body parts must be used in order to remain functional, and the e-brake is similar.