family member who wasnt there "called" police after a dispute between boyfriend and girlfriend in car, what happens?

My buddy is here and we are all from WA.. he ended up in an argument with his girlfriend while driving and she may have been left with a bump on her head from him having to push her off him while driving because she was attacking him during the drive almost causing an accident although no marks were left she then proceeded to walk home from there after jumping out of the car. she got home and her sister is saying shes calling the cops. assuming the cops were called.. what happens then? What kind of charge would it be if any and what might the consequences be? Quick answers would be very much appreciated!