Mycotoxins can be in anything.
http://www.thepoultrysite.com/diseaseinfo/100/mycotoxicosis/
You can have your vet perform the fecal float or see if your state lab will accept samples for testing. You can look up your state here...
Marek's is complicated to say the least, symptoms can be subtle all over the place. Sometimes it can present as loss of balance, stumbling, leaning to full blown "classic" symptoms of paralysis in the legs/wings, wry neck and/or the splits. There are several forms of Marek's and symptoms can...
I agree, the symptoms sound neurological.
How long has your friend had her?
Like @chickengeorgeto says, it very well could be Marek's. Ingesting moldy food is another possibility, but if the other chickens had access to the same foods and they are not showing symptoms, then I would again be...