There are statutes of limitations for all civil causes. The WA statutes are at: http://apps.leg.wa.gov/RCW/default.aspx?cite=4.16 .
The application of the statutes of limitations is sometimes complex. Determining which statute applies and when the time period starts are often subject to dispute.
Debt collectors must abide by federal and state laws. A debt collector contacting third parties (family members) may be in violation of the laws.
Your mother likely can instruct the debt collector to contact her only through writing.
She can review her facts and options with an attorney.
She can review the information available for free at WA LawHelp to see what her legal rights are. The website is: http://www.washingtonlawhelp.org/WA/index.cfm .
If the debt collector is not following the laws, she can report the violation to the office of the Attorney General. The AG's website: http://www.atg.wa.gov/ . While the AG will be helping your mother specifically, if the AG gets enough complaints about a business, the AG may investigate and sue that business.