ml wrote:
"A friend of mine might get permission to put up an antenna or two on
top of his apt building about 30 stories up. mostly flat roof."

Likely the building already has air terminals (lightning rods) around
its periphery. These probably are connected with heavy cable to the the
steel skeleton of the building. If not, that is another story.

Your friend needs to directly ground his antennas to the same system as
the air terminals for lightning protection.

Best regards, Richard Harrison, KB5WZI