Your friend isn't entirely wrong, but he is wrong.
His advice would apply to nickel-cadmium batteries, but those haven't been used to power laptop computers for about 10 years. Basically, it is highly recommended to only charge Ni-Cd batteries in full cycles, where you completely drain them before charging. Failing to do so carries the risk of permanent reduction in capacity.
Lithium metal hydride and lithium ion batteries do not suffer from the same "memory" phenomenon. They are good for a limited number of charge cycles (usually around 1,000) and that's that. Any reasonably modern laptop battery is almost certainly one of these two types.
