Disbelief has more than one meaning. There is a kind of disbelief which is atheism and this is what your friends meant by this kind of disbelief which indicates the non existence of God. Another meaning of disbelief is disbelieving in Prophet Muhammad (peace be upon him) and this meaning is the one that is meant by Muslims when they call someone as a disbeliever. This understanding of disbelief versus Islam does not necessitate atheism or disbelieving in God because Islam is an open system which does not differentiate between prophets. Therefore Muslims reached a consensus that whoever disbelieves in any of God’s prophet while his message is well conveyed to him is a disbeliever even if he admitted the existence of God and believed in all the rest of God’s prophets. Therefore, whoever disbelieves in Prophet Moses is a disbeliever and same goes for whoever disbelieves in Prophet Muahmmad after the Prophet’s message was well conveyed to him.