I have limited knowledge of solar power but it shouldn't be too complicated.
I'd go with a grid tied trickle charge setup. Basically if the batteries start going dead, it switches to grid power and starts charging the batteries up.
Overall, because LEDs don't suck up a lot of juice and are DC powered, it shouldn't be all that complicated. It will be more expensive, but they might see savings in the long run. Hard to say. It would heavily depend on location and how cheap grid power is.