It's not very common but it's not unheard of, the condition's known as monophobia, which you may know already, and yes it does sound like she had it bad, possibly to the extent that it would have had an effect on her health, as persistent worrying does, increasing the chance of something like that happening. I've seen similar anxiety disorders, both self-directed or externally-directed, such as a mother who was convinced her baby wouldn't wake up again if she didn't sing a certain song to him going to sleep. The thought processes that lead to that belief are complex to say the least. One of the difficult things with a fear like that is that it is self preserving, because getting rid of the fear would expose the chance of that thing happening (eg, if she stopped thinking that she needed to sing that song for her child to ever wake up again, then she would stop, which would lead to loss of her child) which results in the putting up of defenses against having the fear dealt with, which you obviously saw first hand.
And yes these things can be very acute, and I imagine can form like synesthesias, where a crossover between two unrelated concepts forms, so the number 7 might always appear yellow. While less common, there's no reason why these can't form with higher function concepts, like the idea of being alone and the idea of personal danger. Of course, there's so many different concepts within our brains that there's a massive number of combinations of things that could result from the same thing happening... and when it is acute, the person, being otherwise of complete rational mindedness, just finds ways of dealing with that thing, or avoiding it, while living otherwise as any other "normal" person would do.