Cancelling Contract When Only One Partner Signs

I'll try to be as succinct as possible. My parents signed what turned out to be a purchase agreement for a water filtration system. On the contract my dad is listed as the buyer and my mom is listed as a "co-applicant". Within the same document there is a purchase order and and a credit application, IDK if the "co-applicant " term refers to the credit application or the overall contract.

There is a cancellation clause in the contract where the entire deal could be voided if the cancellation clause was exercised within three business days. My dad signed and delivered the cancellation documents within the three day window. However, the seller is refusing to honor the cancellation clause because it wasn't signed by my mother. Do my parents have any valid argument here? This is in California if it makes any difference.