re: Knee replacement delayed 2 weeks because of a minor scab?

If I had to guess I would say that the surgeon in this case may have had some specific instructions for the pre-surgical period and maybe your friend broke those rules in the opinion of either the surgeon or the other surgical staff involved. So, in short, yes, it's likely an infection risk. If the scab is right in the area where the incision would be made, you can understand why the surgeon would not want to operate on that area. Their liability is already high enough, so they wouldn't want to slice right through broken skin/scab, if your friend ended up getting a post surgical infection it would be blamed on the surgeon/surgical team/hospital, when ideally...all of that could be avoided by just letting the wound heal first and then operating. They are probably making the best decision for everyone involved.