Step-kids misbehaving....how should they be disciplined?

My friend has been watching her boyfriend's 2 kids (girl-7 & boy-9) while he is at work all day. Her first time really being alone and responsible for them. The girl just does not mind and will do exactly what she's told not to do. Won't eat anything but junk food and constantly makes messes....the boy cusses directly at his sister, pulls her hair... the two together are almost an embarrment to take into public. When dads at home (at night before bed and one full day per week) he does little if any disciplining for their behavior. What should my friend do???