Just take them to small claims court. Have a mechanic put everything in writing as to what the damage is, and why it happened. Oil drain plug probably?
They’re Valvoline, they’re insured, and they do stupid things like this 4,738 times per year average, so she should get her money. If anything, talk to a trusted lawyer for advice beforehand, even if you have to pay. Taking a lawyer to court would cost a lot.