mlittle,
Welcome to ECN, mate!. [Linked Image]
I work with HV stuff like this all the time and it never ceases to amaze me how people seem to treat it with such dis-respect.
However, to answer your question:
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The recommended "safe" distance from powerlines is typically 10' for this voltage. How much does relative humidity affect this recommended distance?
Are you sure the distance is 10 feet?.
Humidity, (or moisture in the surrounding air) would work to lessen the "Flashover" distance, which has happened in this case.
Pure water is a poor conductor of electricity, however, try and find somewhere on this planet where the rain or mist water is pure.
Add impurities and the water vapour will conduct electricity with relative ease.
It goes back to basic Atomic theory, you have to have "Free" electrons to enable current passage through the vapour.
Personally I'd like to know what the MAD (Minimum Approach Distance) was with this.
Other side of the coin, mate, should have this guy even been raised the bed without a Safety Observer watching the distance?. [Linked Image]