... of training and understanding of the principles involved. Really, a brief class in top-rope climbing and belaying would do most sailors a lot of good. Falls climbing masts are far more common than top-rope belay failures when the number of folks involved is considered.
There is no excuse for the belayer becoming distracted. Period.
One line is enough, if it is belayed properly. That's all climbers ever use. But proper belaying of a moving load, whether a climber with climbing equipment or a sailor with turns on a winch, takes practice and training. Perhaps the chossen cleat was not suitable for the task; most are sized for securing lines, not smoothly easing a line under load.
Still, I generally use a second line and place an accender on it (like a Top-climber) unless my belayer is a climber I know to be well trained.
A tether isn't really very suitable for clipping in. Climbing slings and biners are.
Mammut Dyneema Contact Sling at REI.com
I always have a hand full of each clipped on my harness. Much more versitile and faster.
One of the best ways for a begining climber or a small person to belay a larger one is to use a GriGri or Cinch. A child or small adult, anchored to the deck with a sling, could catch anyone.
Video: How to Use GriGri & Automatic Belay Devices While Rock Climbing | eHow.com
Even though they are idiot proof on the catch, lowering requires practice.
But it isn't the gear--she could have simply wrapped the rope around the boom 3x--it's the training. He should have climbed a short distance and had her lower him a few times, to get the feel. The climber is responcilble for understanding his belay.
It just didn't need to happen.