Yes they'll stay there. These things OFTEN go uncaught because it's pretty small potatoes for an operation that's much bigger. Now, I would say don't be surprised if one day they go missing. But it's not like Cash, they can't try and claw it back like if the Govt gives you too much on a tax return. Once those miles are gone, they're gone. So if your buddy uses the miles, then they want them back later, too bad for them.

I recently cleared out my Aeroplan account, down to 0. Then I returned some Old Navy items which I had forgotten I purchased with my Aeroplan credit card. My miles balance now stands at -56. Not totally the same situation. But it's not like they tried to claw back my award ticket, they haven't called me asking to pay for 56 miles (though I suppose I would if they did, cuz this wasn't done intentionally) and next time I earn any miles I'm sure it'll be 56 less than it should and I'm fine with that.

That's the only caution i'd give to your friend, if he's a DL loyalist and they do claw the miles back and he somehow ends up in the -, he's either gotta be ready to "pay em back" or just never credit to that account again.