Where is she dirty? what kind of dirt is the problem?
Does she have access to a dirt bath? Normally chickens need to bathe in dirt. It actually cleans them very well and protects them from mites. They roll in the dirt, kick it up onto their feathers, then they stand up and shake it all out. It makes their feathers nice. Then they'll preen their feathers, which cleans them. If they are able to do this daily, they'll stay pretty clean. You can fill up a small tub with half sand/half fine dirt -- that will work too.
Also, if a few feathers have something yukky, you could try trimming then dirty part off with a scissors (being careful of course).
I'd do those before trying a water bath.