I agree with the prior answers also. I would contact her attorney and offer to refund the money, as you have expressed you would. As the prior attorney mentioned, I would also insist the attorney give you a "written receipt" of the refund that indicates all claims have been waived by both parties in exchange for the refund. I would suspect her attorney would be interested in getting a lien waiver from you and am surprised this hasn't come up yet. I do practice construction law and am located in Puyallup, feel free to call if you like. 253-720-3020
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You need to contact an attorney to help you review this - preferably a construction attorney. If you're willing to give her back the $3,000 and she demanded that you do so, that should likely take care of it (she asked for $3K and you offered $3K - seems simple), but you will want to confirm this in writing.
In the future you should review RCW 64.50 (http://apps.leg.wa.gov/rcw/default.aspx?cite=64.50) regarding construction defect claims, which if you provide the proper notice prior to the job, you would be given an opportunity to cure any defect before she sued you.
I agree with the prior answer. You should consult with an attorney as soon as you are able. It'd be best to review the contract you have in place (assuming you do) between you and the lady who hired you to complete the concrete job. The terms of the contract will dictate how reimbursement is to be collected if the services provided are not "up to par."
Whenever you are "settling" a claim, you need to make sure and have everything in writing. If you both agree to settle for the $3,000, you should make sure you are released from any further claims so she is unable to seek any further reimbursement or "settlement" from you. Like I said before, you should hire an attorney to assist you with the contract review and potential settlement process.