It scares me when people lose a lot of weight in a short time. When I was a kid, three obese women lived down the street, a mother and her two daughters (19 and 21). They were Samoan women. The youngest went on a crash diet. She went from about 300 to about 100 pounds in less than a year. She died of a heart attack, when she was 20 years old, soon after she had achieved her weight goal. Her doctor said she had stressed her body too much with the rapid weight loss.
He lost the weight without the help of supplements and fad diets by combining 2 hour cardio and weight lifting sessions with proper eating habits. Instead of Burger King, KFC, and McDonald’s, he opted for smaller portioned meals of fruit, vegetables, and lean meats. Furthermore he follows a low-carb approach, eating some sweet potatoes but not eating bread, rice and pasta
So losing just over 1kg a week while eating a sensible lowish carb diet and regular exercise seems a good idea to me. I don't think he's putting himself at increased risk. I think remaining overweight would be a greater risk.