6 months is a short time. It was going downhill when she moved in. It's been going downhill for a long time.
In my experience, a change of management can cause apartments to absolutely tank quickly. Some management don't care about who lives there, they don't enforce any occupancy rules...among many other things. The culture can change very quickly. Even run down and aging apartment communities can still be relatively safe and have a good culture. New management can totally change that and apartments can become unsafe very quickly.