Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.
87 octane has more energy in it than 91.
As long as his engine is not pinging -- no harm done really.
and to the person that mentioned "LED" (sic) --- there hasn't been any lead in pump gas for years and years