You can oftentimes categorize the reasons someone does something into three categories: inertia, ideology, and ignorance.
You can cure your friend of ignorance pretty easily. There's at least one study linked here.
Seems like it's not inertia. Your friend has three kids, so it's got to be hard for him/her to day trade.
Which brings us to ideology. Probably the toughest egg to crack. Honestly sometimes it's better to just let them be, because more often that not, with ideology, they've chosen to believe what they want to believe, even if it's contradictory to the facts.
For some people, at the end of the day it doesn't matter how much data you present showing how it's hard to beat the market by day trading after fees and taxes. They believe that they're part of the portion of the population that can beat the market.
ETA: And even if you can present enough evidence to convince your friend that their ideology is flawed, sometimes they're so butthurt that their whole belief system was disproven that they resent you. Because of this, I usually just leave such people alone.