Will a mouthful of laundry detergent harm someone?

I’m not exactly sure how it happened but a group of my friends were together hanging around at my place. And then the guys were just fooling around and told this one guy to drink laundry detergent for fifty dollars he was asking for. They weren’t serious about it, they were going to give it to him but I guess he drank a mouthful anyway. He started choking and gagging afterwards and poured it out. We weren’t in the same room when it happened but we heard them yelling ‘oh you’re a f*ing idiot why would you do that?’ so we went to see what happened and they told us. Will a mouthful do any harm? Will it just make him sick or could it kill him?

He went home afterwards even though some people told him he should go to the hospital to make sure he was OK. All he said was that it was fine and said it wasn’t enough to do any harm. He laughed and said we should worry if he had drank the whole glass but not from a measly mouthful. He coughed a lot for the rest of the night and said his throat hurt a bit but that was it.