With her experience I am surprised that she has so much difficulty in comprehending the dictation. While you have made what I think is an excellent analysis I am afraid, the solution is not so apparent.
I suspect, the steno has to work on her vocabulary and that means she has to read extensively and also use a dictionary and thesaurus too. I also believe she needs to tune herself to the bosses style and requirements. Perhaps she will also need to focus on technical language used in the dictation.
She is welcome to read the following Inspirational and Motivational Blogs to get a feel of language.
www.actspot.wordpress.com - Daily Blog
www.poweract.blogspot.com - Weekly Blog
I hope my inputs will help the lady. My best wishes to her.
A practical solution, is to ask her to carry a small tape recorder and tape the dictation, so that if there is a doubt. she can revisit by listening to th recording and get the work done.
Long term perspective, she would need to undergo a language neutrlisation workshop to be able to comprehend and match to your organisations needs.
You can approach any good trainer/institute to have them deliver this program