Air bag system on 1991 ford mustang

Hi my friend was in an accident and her boyfriend was driving and not wearing his seat belt. well when they had the accident the air bag never deployed. my question is, Does the seat belt need to be fastened for the air bag to go off in an accident or does just go off no matter what? if you could let me know it will keep her from getting into a massive lawsuit over people who she thought were friends commiting fraud and lying. thank you. Will