I did this for years...neglecting to take my preventor inhaler and just using my blue one. Always thought I had pretty mild asthma because I was physically fit and the blue one seemed to 'control it' fine.
Then one day it didn't.
I started wheezing just while lying down watching a movie with my Mum. Took my blue inhaler, nothing happened. Took more and more puffs but I just kept getting worse. Eventually I realised it was getting serious; I felt the classic 'sense of impending doom', I was literally gasping for breath but it wasn't coming...told my Mum to call an ambulance and that was the last thing I remember. Apparently I collapsed seconds later.
Ended up in ICU/HDU for nearly two weeks. Was on the verge of getting ventilated when I first went in, due to how high my carbon dioxide levels were, how low my oxygen levels were (mid 70s) and the fear of brain damage/death that comes with those.
When I eventually recovered they termed it a 'Near Fatal Asthma Attack' - and told me it was entirely down to me not using my preventor inhaler. One doctor was really scathing...said how irresponsible it was to come a coin toss away from death, leaving my family behind, because of something that takes 30 seconds a day.
Since then I've literally never missed a dose. I know I probably read stories exactly like this and ignored them, thinking it wouldn't happen to me, but you should really press your girlfriend to make it a priority to take her preventor.