Stress, lack of sleep and colds/viruses all seem to be potential triggers. Help her avoid all 3. Keep your girlfriend well hydrated, rested and if you're a cook, some homemade chicken soup is the best for antiviral and anti-inflammatory propeties. Made with fresh veggies will only help:
Here's a terrific recipe and explanation of chicken soup's healing powers...
Some supplements that I believe have kept my husband cold and virus free for eighteen months are: curcumin, vit. A,B, C, D, and E, resveratrol, quercitin, glisodin, omega 3 and 6 oils, and lots of anti-oxidants. We're also rather OCD about handwashing now. Just hot water and soap after being out in the world.
Good luck to you both.