Clarifying Full Amount Letter from SLS

Hello,I watched your videos a little while back and you made some points about having a letter to help in a defense vs Student Loan Solutions LLC. You are one of the few I have found to have knowledge of this kind of lawsuit. A brief history:My ex-wife and I took out a Bank of America private student loan (in my name) in 2007. My mother co-signed.The loan was deliquent for many years.In 2018 they sent a letter to me via WIlliams and Fudge, requesting the full amount. No due date was given, just \\"pay up\\". They did not follow up.In October 2023 they informed my mother that she is being sued by Student Loan Solutions via the law firm, Goldman and Hull (i think).This is in California.The loan with interest is around $42,000. They are suing for $15,000, oddly. They say the older \\"payments\\" that were due have expired statute of limitations, so this lawsuit is for the last 4 years of hypothetical payments (do PAYMENTS have statutes??).This feels odd to me.I suppose that\\'s my end-questions here are:Is my letter valid and her #1 defense?Should any run-of-the-mill lawyer defending a civil case like this understand how this works or do we need some specialty lawyer?Thank you for your timeRick