In his case the board supports 1.5V since he's already running DDR3 modules(they usually run at 1.5V or 1.65V)... so the DDR3L should work.
The other way around wouldn't have been possible. If he had DDR3L memory on a motherboard that supported only DDR3L, adding DDR3 wouldn't be possible.
But as stated earlier mixing and matching can lead to incompatibility or instability.