Unless your friend is a dependent adult and there is some court order that grants her parents conservancy over her into adulthood, she can walk out the door today.
Her parents MIGHT hold some civil liability for certain actions so long as they claim her as a dependent or are providing for her board and care (if she is a student, for example), but that will certainly not hold in all situations and certainly does not prevent her from rejecting their assistance and being on her own.
If she does not like it at home, she can leave. Of course, she may not have a right to anything more than the clothes on her back when she walks out the front door, but that is a choice she might have to make.