Yes, understood. Knowing this now and given the distance, I wonder if Pheobe intended on returning? I wonder if she actually had a one way ticket, and the housemates realized this as they neared the airport--maybe after the call with the boyfriend--and that caused some sort of rage event?
If the housemates expected Pheobe to chip in, and Pheobe was trying to have some money for travel, then that really goes a long way towards explaining her unkempt and starving appearance at the grocery. I don't think it has to involve drugs at all. It really could just be a struggling kid trying to break free, and an adult (adults?) with a violent anger problem furious that someone else was "getting away" instead of being under their thumb.
From the little information we know from her statements and appearance, and the dreadful living situation at the place where TB and FW were living, I would not have expected her to return there, but to try to cash in the return ticket and make her own way in Perth.