Ignoring your comment about how they are parents that already 'push the envelope' I feel that a surgery on a child is out of the norm enough to justify some changes. Turn it into a learning opportunity for the group. "Johnny has surgery and can't walk very well so let's help by bringing him a few toys, and going to him to play today".

Can he go in a wagon for your walks?

I wouldn't suggest this for colds and illnesses that are a rather regular occurrence but this is a young child who is having surgery. His family has booked a week off to be home with him. Odds are they can't manage more. Odds are after a week at home he will be thrilled to rejoin his friends, even if he can't quite keep up.

If after trying it out for a few days it really isn't working, then inform them they need to keep him home.