I'm with Brent on this...
Man up and talk to her. Do it now, do it later wehn she calls, whatever, but don't let any emotion come into it. Never do it as you're leaving the job.
I've done it a few times, I normally tell then that despite my attempts at providing a serive that is not only the best we can do, but also at a vlaue that our clients appreciate, we realise that some people will always want a higher level of service or have higher expectations than we are able to achieve/deliver. I then state simply, not in an accusational manner, that over the last x times we've worked for you, you have felt we didn't do a good enough job, our price was too high or whatever they are complaining about.
Gently give them an honest suggestion of another service provider that will better suit their needs. Do this in all honesty, and try and match your referral to her complaint (price/quality/whatever). You may also want to warn them as I have.
Some politely move on, others bitch and move on, and a few, realise they dont get to wear the pants in our contractor/client relationship and really are happy with your service and stay on as loyal clients.
John