However, another idea would be:
The girl goes to the garage as would happen in the normal opening, but then instead of being killed, the girl jumps because someone grabs her shoulder. The girl turns and realises that the hand on her shoulder is her boyfriend's best friend. She sighs and hugs him saying 'omg i was so scared!' and the friend says 'what are you on about, aw, come here its ok' (because she is crying). The friend then gives a look signifying he is the killer or knows something (over her shoulder) or he sees the mask and smiles or something showing he is the killer.