Or just send him some videos of ISIS rounding up groups of Iraqi's and massacring them or Boko Haram kidnapping Nigeran schoolgirls or aid workers getting beheaded or go old school and show some pictures of people in Sierra Leone with their long/short sleeves.
Or just ignore it because sometimes there is no reasoning with these people. I guess some people just don’t want to know the realities of what is out there and why militaries are needed.
As an individual, you do not have to justify your career choice to anyone but yourself.
The thing to be aware of, and it's certainly worth thinking about, is members of the UK armed forces must be publicly apolitical. Even if you don't believe in the ethics or politics of a particular operational task, you swear your oath of allegiance to serve wherever sent.
Often we get people wishing to join the UK "Armed Forces" (clue in the title) who ask if they can serve in a branch that only undertakes humanitarian or environmental operations.
Fact is, the military does undertake tasks such as disaster relief, but as 'first responders' and in the aftermath of a disaster, often there is a requirement to also provide security, medical aid, food supplies, restore power/water/infrastructure, disposal of the dead, etc in a hostile environment to enable the civilian aid workers to be effective and operate safely when they arrive.
In other words, "Who else ya goin' ta call" when it goes pear-shaped and the civilian infrastructure is equally affected by the disaster?
Likewise warzones. The civilian populace are victims too - who do you send as a first responder?