So, you've persuaded nagged your father in law into wearing a helmet because he banged his head when he wasn't (I assume) riding a bike, despite the fact that he's managed to reach the grand old age of 80 and never worn a helmet, so has managed not to die whilst not wearing one for quite a long time. Then you let us know that it is debatable that one is more likely to bang your head cycling than walking, particularly it could be suggested for an 80 year old, I'm guessing however that you're not suggesting that anyone, even your 80 year old father in law, should wear a helmet whilst walking, even though as you say yourself it would be a good idea for your father in law.
Can you not read what you're typing?