A couple of options:
• Create the flyer with a blank area and import it into Word as a background (Header or Footer). Let the client add the text in Word.
• Create a PDF from the artwork leaving blank areas for the text. Open in Acrobat and create a form type PDF with editable areas.
If it's not simple text, then they'd need some more sophisticated software like InDesign, Quark or even the open source ones.
One of my clients asked for something similar. They couldn't understand why they can update their website but not their print-based items, and I suppose they had a point.
There probably is another option that I'm not aware of, but I wouldn't recommend it as it'd put me out of a job :icon_biggrin: