Is it true what my friend says that when someone says something crazy cause of psych issues they dont remember it later, a neighbor shouted across the street to me one day and told me to die, when she was asked about it she said she didnt do it?

PTSD in its simplest form involves forgetting or misremembering details of a traumatic event. If this person was experiencing major physical or psychic trauma at the time, then it's possible she's forgotten her poor choice of words. However, people with personality disorders are also capable of denying the truth and outright lying. If you value the relationship, talk to her about the incident.

Many explanations are possible for above scenario from simply misunderstanding the message to possibly having medical, psych or being simply intoxicated that will cause premises for sending hostile messages with no recollection later on.