It will help to frame it carefully in your own experience, and in the context of "what's the best way to test", not "here's what will happen" as if you were omniscient.
Quoting from peteforde's response above: "a startup is a temporary business structure that exists only to prove or disprove a hypothesis about a market opportunity in the fewest number of steps (time, money, resources)."
You have a gut feeling that it's going to go poorly, based on some solid experience that he lacks. That's fine; it doesn't actually mean his business (or some pivot from it...) won't succeed. How many stories have you read where the great idea is just a few twists away from the shitty initial plan?
Just help him find a way to spend less than two years, and less than his life savings, to find out if his idea can get traction. You have concerns about risks he's not considering -- help him consider them (and address them).
In this kind of discussion, with each risk discussed (and each step his idea/plan is refined in response), you're helping him succeed, not cutting him down by being a know-all naysayer... that's what friends are for.