non kosher grill

Question

i was a witness of a situation yesterday.... Our family came to a friends' house for a bbq so we told him to foil his grill so we can them grill fish on the foiled grates. He did just so, but when it came to grilling burgers(glat) he removed the foil and scraped his grates and proceeded to grill our burgers on his grill, insisting that the grated got red hot while under the foil which kashered it. we did not eat those burgers :( were the burgers indeed kosher to eat? Thanks!