This is brief as I am at work but...
I'd encourage your friend to at least have a meeting with his healthcare team and understand what the actual diagnosis is.
My AH told me he was diagnosed with Bi-Polar when we met. Throughout the years I learned a lot about bi-polar and it didnt really resonate with AH's behaviours. 'Rapid cycling' is only 'rapid' in compariosn to bi-polar I or II. From memory its four or more changes in mood during the year. My AH used to change much more quickly than that.
Turns our his bi-polar was actually borderline personality disorder. There was no bi-polar. Complicated by addiciton and GAD.
I never would have found out had I not eventually insisted on meeting with his psych - as so many things didnt add up. Now i know. My counsellor encouraged me to do this - said we can take 'keeping our side of the stress clean' too far sometimes. KNowledge is power and all that.
But at the end of the day - as everyone says - the cause doesnt change the behaviour or how one of being treated by their partner...