DeeDee brings up important points - there are aspects within her profile that may interfere with writing beyond the ADHD - and these issues should be addressed too. Could it be that the meds actually did help with concentration, but that the other underlying issues that were not addressed are causing the havoc? I agree that if she is not getting the support she needs in terms of accommodations and or perhaps private tutoring, perhaps this is one step she can take to help.
Regarding the meds - if they are truly not working, I would seek medical help to either change the med or adjust the dosage. Generally one can stay on the same dosage for quite awhile, but it is possible that one can develop a tolerance and require higher doses. This usually happens over a number of years - not months or weeks. If your current doctor is not an expert in ADHD - either a psychiatrist or neurologist with a specialty in the area, you may want to consider seeking a second opinion. Many general practitioners may not have the experience to help with more complex cases - especially doctors that treat adults - they probably don't deal with ADHD much at all.
You might also want to check out millermom proboards - it is a forum that specifically addresses ADHD and LD. If you post in the ADHD section, you most likely will hear from very knowledgeable people that can give you further direction.
http://millermom.proboards.com/index.cgi?board=general
