Your daughter’s challenges echo those of many struggling writers, and while there are no quick and easy fixes, there are technology resources that can help.
This is a common issue for adults (and kids!) with dyslexia. It can be particularly challenging when you have a word spelled correctly, but your usage is wrong.
Dysgraphia (difficulty with fine motor coordination resulting in difficulty writing) is difficult to treat. It is unusual for someone to develop writing problems at age 17.
There are many websites and software programs that can calculate readability for you. For shorter pieces of text, you can use one of the many online readability programs available listed below.