I presume you're asking if there's a problem if the buyer works with the listing agent? It's not illegal. It's not ideal for the buyer. The listing agent probably won't be as helpful and conscious of the buyer's best interests because they have no reason to be.
They just need to do the bare minimum within their legal and ethical obligations. Their primary duty is to make the deal work to the best interests of the person they have a contract with and that person IS NOT the buyer.