I had this exact same problem. I eventually ended up in emergency and was diagnosed with Irritable Bowel Syndrome (IBS). I had severe pain - so much that I could not even sit up - lay down - or even sleep. It hurt..... I was told to eat fibre tablets or put fibre into your diet. I attempted doing that, but the fibre pills I bought tasted absolutely gross. A few months later, I went to a chiropractor for other reasons - now I am not one that will promote a chiropractor, however a few of the bones in my lower back were out of place - he aligned my body and I crapped about 5 times the next day (I know too much information). It still happens when I see the chiropractor. Upon speaking with my chiropractor about this, it seemed that the "signal" from the brain to the muscles that digests my food is being intercepted by the bones that were out of place. I know it's weird - but it is worth a shot....
Also I started taking probiotic pills (they must stay in the fridge) - after about a week I noticed a huge difference in my bowel movements. You can private message me with any other questions you have if you want to - as I've been dealing with the constipation issue for about 2 years now....
I have a friend I work with who has the same problem. No one ever diagnosed her with anything. She was told to take laxatives in order to have a bowel movement. I do know that if you do not have enough water and fiber in your diet, it will cause some constipation, but this does not quite sound like the problem in your girlfriend's case. Tell her to try dietary teas.