I'm late to this thread, but I'll give my inputs on this particular scenario. I know this may discourage or even give doubts to people who are considering the firearm as their method of choice (I'm not deterred personally), but after studying what went wrong, I could conclude several factors. The first one being that she flinched and jerked the gun as she fired which resulted in not hitting the vital areas (brainstem and then hitting some other parts that result in failure and further suffering), then of course aiming under the chin (absolutely NOT recommended and contrary to what the movies and media show) results in the bullet hitting somewhere where it isn't imminent death but likely survivable but with permanent disfigurement and damage. Finally, being able to steadily aim and being in a clear, coherent state of mind (more than enough to focus and have enough motor function to aim properly) is important too, hence why there are some people who even recommend against being too heavily under the influence of drugs or alcohol as that may be counterproductive towards the execution/attempt of the method.
Finally, the firearm method is not a method to be taken lightly and can be worse (permanent severe debility, ending up as a vegetable unable to CTB and then forced to live in agony for the rest of one's life) if one fails, thus it is highly recommended to thoroughly and properly do one's research for this method (or any method in general), and also not to attempt when one has any hesitation or compromise to their ability to carry it out.