Is melting aluminum cans actually dangerous to breathe or is that a myth?

Your buddy isn't wrong, but it's not quite as scary as it sounds for plain aluminum. Clean aluminum itself doesn't give off much in the way of fumes at melting temps. The problem is everythign else that comes along with scrap. Paint, plastic, oil, whatever was inside that can, all of it burns off when it hits the melt and you do not want to be breathing that. This is why I stopped tossing unknown scrap straight into the crucible years ago.

The big one to watch for is zinc. A lot of die cast stuff looks like aluminum but isn't, and zinc boils right around where you're melting. Breathe that vapor and you get what people call metal fume fever. Feels like the worst flu of your life for a day or two. Won't kill you but you'll regret it, and it can hit harder each time. Brass and zamak parts are the usual culprits, so learn to tell them apart from real aluminum before anything goes in the pot. The float test and a scratch test get you most of the way there. Cans are thier own issue. The paint and the thin liner inside burn off and make a bunch of nasty smoke. When I melt cans I do it outside, fully open air, not a half open garage door with me standing in the middle of it. A half open door in a garage is basically a smoke chamber with one small exit.

As for a mask, get a proper half face respirator with P100 filters rated for organic vapors if you're doing anything smoky. The cheap dust masks do nothing. They're not expensive and they last a good while. Also melt on a day with a little breeze and position yourself so the smoke drifts away, not toward you.

Honestly for weekend ingot pouring, clean scrap, outdoor melts, and a decent respirator covers you. Just don't melt anything you can't identify, and if the melt starts smoking heavy, back off and let it burn out before you skim or pour.

Agreed with everything above, especially the part about identifying scrap first. I learned that one the hard way with an old lawnmower part I assumed was aluminum. It turned out to be zinc alloy and the smoke it gave off had a weird sweet smell to it. Woke up the next morning feeling like I had a fever and chills that lasted about a day. Now I scratch test anything with a file before it goes anywhere near the crucible. If it scratches bright and shiny it's probably aluminum, if it's dull gray and soft I set it aside. That one lesson was enough for me.

Glad this came up because I've been doing something similar with old window frames and cast offs from my dad's shed, just in a steel pot over a propane burner out on the driveway. Never thought twice about the paint until now. One thing I'm still fuzzy on, is it okay to burn the paint off first in a separate fire away from the melt, or does that just move the problem somewhere else? Also does charcoal make it worse somehow, like does the smoke from the fuel mix in with the fumes and make it harder to tell when something sketchy is burning off?