They would need a gateway of some sort to link any e-commerce site with their bank. A merchant account is only one part of the equation.
That allows them to accept credit cards but you have to transmit the transaction information through the Internet to the the issuer of the merchant account. You have to pay a gateway service for this. You should be able to use their account through PayPal Pro. Check with PayPal under the "already have a merchant account" section.