@redstaR - Arthroscopic knee surgery is a type of keyhole surgery, yes. If the surgery is scheduled early in the day, your friend shouldn't have to stay overnight. That’s one of the big benefits of these new minimally invasive surgeries.
Speaking from personal experience though, they may begin the keyhole surgery and discover there's more damage than they expected and have to change to open surgery. This requires longer healing time, longer stay in the hospital, heavier painkillers, you get the picture. This happened to me when I had to get shoulder surgery. I went in expecting to get a "keyhole" sized scar and walked out with an eight inch long scar. At least it was successful!