Your son's friend committed murder and then suicide. Your friend's son committed suicide. As you mentioned, they were at fault.
The means they used are immaterial. Knives, extension cords, ropes, belts, are found in most households and all are used in murder and suicide.
The son of the folks who lived across the street from us committed suicide with a pistol. One of my younger son's friends hanged himself.
I fellow I knew in college murdered his parents with a rifle. A family who were close friends of our were murdered by their son who committed arson.
There were more than 48,000 suicides in the U.S. in 2018. The difference in the total number committed with a gun and the total number committed by other means was 520.
Murders? In 2014, the FBI reported the lowest murder rate since 1957. The rate jumped in 2015 and 2016, but declined again in 2017 and 2018.
It's estimated there are more than 100 million American gun owners. If you add the firearm-related homicides and suicides together, 99.96% of those owners didn't kill themselves or anyone else. And that only works if you assume that every homicide was committed with a lawfully possessed gun.
I hate to say it, but after years of anecdotes being paraded as causes for action, I am losing my capacity for empathy. I regret your loss but accept no responsibility for it; it has nothing whatsoever to do with mine or anyone else's guns.