Thanks for the extra information, it certainly clears up a few questions for me. There are a range of Psychiatric 'scenarios' in which a person can become dangerous and while I'm only privy to a limited amount to information there are some comments I can make here that may be helpful.
Given the apparent lack of actual conflict between your friend and this person I would presume that a person acting in this manner (assuming that there is in fact a Psychiatric cause) is either
1) intensely over-reacting, that is, an inappropriate pathological behavioral response
2) confused or mistaken about events that have transpired, that is, delusional
1) Broadly speaking maladaptive behavioral responses (in the absence of delusions) can be common to some of the personality disorders (PDs). PDs are not psychotic as such so they usually have a realistic understanding of what is going on around them. However, they way they behave is often extreme, unstable and rigid particularly in regards XXXXX XXXXX interaction. People with Borderline PD, for example, can respond aggressively and intensely to minor insults, or transgressions due to their poor ability to regulate their own distress and emotions. It may be that this person is offended and unable to manage their anger and behavior in an appropriate way.
2) Delusions can occur as part of psychotic illnesses and also at times as part of mood disorders, however delusional thinking within mood disorders occurs during periods of acute sever illness so I would presume that delusional thinking as party of psychosis
is more likely. Delusional thinking can occur at times of sever illness (think Psychiatric ward) or with lesser intensity at other times. It may be that this individual has developed some delusional beliefs around your friend and is reacting within that context.
Regardless of the underlying cause your friend is wise to contact the police. While individuals with Psychiatric Illness or not necessarily more aggressive or violent per se, they can be less predictable, volatile and reactive.
If I had to put my two sense on something (given the lack of information I have) I'd be leaning towards delusional thinking. Psychiatric medications are more routinely used in the treatment of Psychotic illnesses and given the references to medication it may be that his condition is deteriorating. PDs are relatively stable (that is there behavior is usually quite consistent even when disordered) whereas Psychotic symptoms can vary intensely as a person moves in and out of acute illness.
I'm not sure if this is the kind of reflection you were looking for. If you have further questions or I haven't quite responded in the manner you were expecting please let me know and I'll respond further. If not....I wish your friend the best of luck!