If she'd like to try a liquid/predigested diet to give her colon a chance to rest and heal, check out the book IBD Remission Diet. It gives instructions for using elemental shakes, healthy oils, broths and juices. In one test, an elemental diet was as effective as steroids in inducing remission.
I'd also recommend you check out Breaking the Vicious Cycle, Listen to Your Gut and The New Eating Right for a Bad Gut for various IBD diets. Don't go into this blindly. Do your research, then it's trial and error from there. Also, don't expect overnight success. It takes a long time for the body to get ill to the point of IBD. It's going to take a long time to truly heal.
In addition to dietary changes, she should also look into probiotics and supplements that are healing to the gut like l-glutamine, which can be very effective in controlling diarrhea.
Kudos to you for helping your girlfriend.
Diagnosed with ulcerative colitis spring 1999.