According to Detroit’s own records, police have knocked 44 people unconscious in the last 5 years with neck kneels. You fall unconscious when the lack of oxygen/ blood to your brain is limited, and continued pressure/ lack of oxygen and blood could definitely kill you. Yes, adding stress/ heart issues/ possible drugs could make it happen faster.
Keep in mind police would be trained in the necessary pressure points, unlike your friend. But I wouldn’t try again because him accidentally shifting his weight could chas v’shalom cause severe injury.