If she is using CS6, you can buy InCopy for 250 dollars
If she is using Creative Cloud, you can get it for $20 a month.
this will require working with her to set up an InDesign/InCopy workflow.
Only InCopy but it would require that the InDesign user export the content for that purpose.
InCopy CS6 and later is available as part of Creative Cloud for $20/month.
But…if it’s CS6 and only needed once in a while, you can still go the $20/month route on a month to month deal.
For buying a month here and there as needed, without a yearlong commitment, the cost is 29.99/month.
You say that you want an Adobe product, but you may not be aware that
there are other products that may work for you as well, in particular
DocsFlow or WordFlow (EmSoftware -- no affiliation to me!)
This would allow your designer to export the story to Word or GoogleDocs
where you make your changes -- and he or she then simply updates the
document in InDesign with the press of a button.
Worth looking into.
+1 to both of those. Excellent products.