Help with a limited power of attorney?

My mother and I have a joint credit card account that is long past the statute of limitations for both being sued and for reporting and she is being harassed by collectors still. I already have all the information I need to dispute the debt, ask for validation and send a cease and desist, what I don't have is a power of attorney to represent her and make the collection agency deal with me only. I want to handle this for her and need her to sign a limited power of attorney so that I can do so. Can anyone tell me how it should be phrased? I live in Texas and she lives in Michigan.