Bob and I were descending from a weekend in the Mount Cook area where we had been climbing. We had exited out the end of the Boys glacier and were stepping off the snow for a small section on loose rocks along the ridgeline before we would return onto the snow. We were still wearing crampons. I turned as Bob yelled to see him cartwheeling down the ridgeline, out of control after tripping. He stopped only a foot short of a 60m bluff thankfully. He had badly sprained both ankles, and was bruised, sore and in shock. We had about 8hours of technical ahead of us and I knew there was no way we could get through it given he was in too much pain to stand up. I activated the PLB and we waited on the ridgeline. The Mt Cook rescue team arrived by chopper and winched us both to a nearby hut where we were then flown to the village and Bob received medical care. So glad I had my beacon with us that day!!