It's great to hear that the climbers involved in this accident are going to be ok. Scary.
There are a couple points that I'd like to make about this scenario. First is to add to RG's comment about redirects. This set up must be built like a top rope anchor because that's just what it is, it's got to be able to support the forces/weight of the falling climber + the weight of the belayer, both are hanging frrom it when the second falls. You would never make a slingshot top rope anchor with a single piece and a redirected top belay must be viewed the same way; strong, equalized and redundant.
The second point is that a redirected top belay is not the correct technique for the situation, it is less good, a lot less. Because of the significant weight difference between the belayer and second even a bomb proof redirected belay presents significant faillure potential. My understanding is that the second outweighed the leader by nearly 100lbs. This means that with even a small amount of slack in the system the falling second will be pulled quite a distance towards the redirect/anchor. Assuming that the leader used some sort of sling to tether themselves to the anchor they were probably a few feet, at most, from the master point. Not a stretch to assume that the belayer will likely be smash into the redirect making loss of control of the brake strand a significant possibility.
There is a lot of debate out there about whether belaying directly from the anchor or using a redirect is preferable. It seems to me that, especially in this scenario, a belay with a munter, gri gri or plaquette style device directly from the master point is infinitely better than a redirected belay from the leaders waist and would have prevented the accident.
From some of the posts it appears that folks might not completely understand how these techniques work. Please, please, puh-lease figure this stuff out before your life and that of your climbing partner depends on it.