I have guns.
I can trade for what I need.
If the situation is truly desperate, people will be willing to trade a substantial amount of goods for the ability to protect themselves (and what they have left).
You have food. I have guns.
Protect yourself with that granola bar and water bottle, when the next armed person comes along.
Your friend is sorely mistaken, in that he believes people who are smart enough to stockpile food and supplies will not have ways to protect those assets. I love guys who say stuff like that, the whole "I have guns I'll just take what I need."
There is no such thing as a one man army, people who think they can only rely on their stash of guns and ammo after SHTF will be among the first to perish. Probably trying to "take what they need" from someone, or in a shootout at Walmart trying to get that last case of water....