Sure deposit and security deposit

[Katrien Filez]

A friend is currently renting a 3 bedroom apartment and paid a sure deposit. Two other people are renting the 2 bedrooms my friend doesn't use and they paid a security deposit to the landlord. Those two people are leaving soon and want their security deposit back, but landlord says no because security deposit is per contract binding till end of contract, which is in a couple of months....As a future landlord, I was wondering if this is legal? And what would be a better way to handle this kind of situations?