Great place to train. What I suggest is you wear Rune legs, Body, and Kite. There are 4-5 skeletons there and they WILL attack you, but if you run around they usually get stuck on a rock or corpse so you should be fine. If you have the armor I listed then it will take them a while to eat away at your lifepoints.
In fact, the Bone Yard hosts the most ingenius way to train prayer, provided you have the right levels. First, 43 prayer for Protect from Melee, and second you need 8500 Dungeoneering tokens to buy a Twisted Necklace. The Necklace revives a Prayer point for every bone buried, so you can walk around with Protect from Melee on for as long as you want - well, if you keep burying bones.
I recommend some prayer gear if he's doing the Melee method, Otherwise he'll have to be very fast in rune with the bird necklace, I think it has a negative prayer bonus?
It has a +9 prayer bonus.