Yes, A can sue B on this, because this debt does not grow out of the accident--e.g. it is not from someone suing over property damage or personal injuries--but rather from the loan/financing agreement (what you call the "lien"), which is a contract which both of them signed. Both A and B obligated themselves equally to pay the amount owed on the lien, and thereforeA can insist that B contribute to the payment and sue B, under the loan agreement, if B does not.