I don't know where you heard/read that bi-polar was predictable, because it's actually quite the opposite- hence the name, bi-polar meaning two extreme opposites. If you think of mood as on a scale of 10- 1 meaning suicidal, 5 meaning a functioning neutral mood, 10 meaning deliriously happy. Those without a mood disorder will only dip to under about 4 when the situation warrants it- i.e; death of a loved one, serious debt problems, serious illness of a loved or self, death of a pet (for examples)- and will only go above about 8 when the situation warrants it- i.e; win the lottery, birth of a child, dream career obtained, winning a TV show (for example). On a day to day basis, those without a mood disorder are around 5-6.
With a mood disorder, such as bi-polar, the scale can fluctuate from 1-10 without the situation warranting it.
This can be heightened and/or antagonised with delusions, hallucinations (auiditory & visual) and paranoia.
Thus, the illness itself is very unpredictable, yet triggers can be somewhat predicted, but this can take years of a sufferer making notes & making observations on their own episodes & extreme mood changes. This can made difficult by the fact that when someone with bi-polar is going through an episode, they don't neccessarily understand their own thought patterns & behaviours.
One of my particularly bad episodes ended with me in my underwear waiting for some random man to pick me up. I'd met him in a nightclub the week before & called him telling him I was replacing a member of a well-known girl group. Luckily, my mother intervened & I ended up with something shot in my be-hind. I wasn't drunk nor on drugs- at the time I was also counting knives (I was paranoid about them being taken by a burglar).
I was diagnosed with bi-polar soon after this, but this wasn't my end diagnosis. Severn years later, I was diagnosed with schizoid- avoidant personality disorder; however, characteristics of my mental health illness changed and I agree with my final diagnosis. Schizoid-avoidant personality disorder, borderline personality disorder & schizoptypal personality disorder can all show characteristics synonomous with bi-polar. Yet where a doctor will instantly medicate for bi-polar, personality disorders don't react to medication in the way that someone with bi-polar would. Additionally, someone who has a personality disorder is more aware of their episodes than a bi-polar sufferer. While I was usually aware of what I was doing, I felt as if I couldn't stop it from happening.
At the time though, I showed all the signs for bi-polar, but the medication numbed me out. I would sleep lots and not feel anything in my waking hours. I still display signs of (unmedicated) bi-polar; my moods fluctuate very frequently, I can get hyper-active to the point of insomnia for days on end, I'm very creative (which is associated with bi-polar & other mental health problems- look this up!) and can have my 'reckless moments' (which are now tamed and more likely to be throwing myself down a hill on my bike, rather than sleeping with strangers as it was in the past!).
I don't take medication, as it's useless.
The best help I got was CBT- cognitive behavioural therapy. Has your girlfriend tried this?
Whether she has bi-polar or a personality disorder- CBT will help her immensely. It works with changing the thoughts associated with negative feelings and behaviour. Its about mood management and understanding triggers to extreme negative/positive emotions and observing our behaviour towards the triggers. It also encourages planning strategies, to prevent low mood plunges.
A relationship with someone with bi-polar is going to be difficult, yet no relationship or person is perfect. You really love her by the sounds of it- the best of luck to you both.