When I was working satellite operations, our ground station antennas had a beamwidth of less than 0.1 degree. If they could create an antenna capable of handling that kind of power with a 0.1 degree beamwidth (preferably less), then the beam would spread to about 436 miles wide traveling from the moon to the Earth.

beam spread = distance * tan(beamwidth)

I personally don't think much of the professor's idea. Unless the power stations are located in one of the few locations on the moon that's always in daylight (very prime lunar real estate), then you'll have to build multiple power stations to try to avoid the 2 week long lunar nights. Even with a very tightly focused antenna, some of the transmitted energy will be lost.

About the only advantage I see from his idea is that a lunar power station wouldn't be limited by the size and mass constraints that a solar power satellite would have. I'd still prefer a power satellite at geosynch. It'd be in sunlight over 99% of the time, beam spread would be much less, and it wouldn't require building a hugely expensive lunar infrastructure. Building on the moon would be cool but I just don't see the kind of financing necessary for his project to ever come forward. Even the economics of a geosynch solar power satellite aren't favorable at the moment.