A friend insists on putting his car in park at stop lights to save brake wear. Any validity? I think dangerous in case he needs to react in traffic.
Does he think holding the car still is going to wear out the brakes? That’s crazy.
I agree but he won’t stop and I think it could actually be dangerous. I would like to prove that it doesn’t help the brake wear and possibly dangerous.
No brake wear sitting there stopped.
The huge delay in getting started at the green light is rude and inconsiderate to the number 10 or 11 car behind him who doesn’t get through the light as they know they dang well should have.
Make him learn what “friction” is and how brakes actually work.
You should also have him price out the difference between having the brakes done and having the transmission replaced. Ask which one he’d rather make do more work at stop lights.
The OP’s friend has got some very strange ideas.
Does he also avoid wearing his eyeglasses so that he doesn’t wear out the lenses?
As has been said, keeping the brakes applied to a stationary car does NOT impose any wear on the brakes.
And, as has also been said, putting the car into “Park” while at a traffic light makes it impossible for someone to react quickly to emergent traffic situations and is just plain discourteous to all those people waiting in back.
Like Ron White says, you can’t fix stupid.
Nothing you can say is going to change someone’s mind who is doing something so stupid. The smartest thing you can do is refuse to ride in his car until he wises up.
You should find some new friends. 
Wait until he sees a speeding Excursion or Suburban coming in his rear view mirror that shows no signs of slowing down, or the driver’s head for that matter(because they decided to reach over and grab something from the passenger seat and can’t be bothered to look at the road).
Brakes only wear while applied with the wheels moving. Once the wheels have stopped moving, there is no wear even though the brakes are still applied. Tell your friend that if you really wants to reduce brake wear by shifting into park, he is doing it backwards. He should shift into park while moving to brake (and break) the car. Once stopped, he can apply the brakes and shift back into gear. This way will avoid brake wear. I suspect that the resulting transmission “wear” will be more expensive than the brake wear, but…
Please notify the person that buys your friend’s car of the cruel and abusive treatment the transmission has been put through. Also encourage your friend to read the owners manual for proper use of park. He should be putting the emergency brake on and shutting off the engine just to be real safe at every light. 