It was mentined that in full PVP mode everyone in an area effect, friend or foe, is affected by the spell. What if the spells description specifically states it only affects enemies?

The description for the spell "slow" actually says only enemies in the area of effect are affected.

With full PVP rules does this mean everyone is affected?

((apologies made for incorrect/mixed-up use of effect/affect, I can never get it straight which to use when))

If a spell states that it doesn't affect friendlies, or that it only affects enemies then that's what happens. In NWN, full PvP just means that spells like fireball will affect everyone. If the PvP mode of an area was set to Party, you'd never be able to damage friendlies.

Slow only affects hostiles.