Do you mean on Trulia? Call Trulia. You will want to make sure that when you entered the listing on the MLS that you checked the box authorizing it to be syndicated on line. I believe you can also manually list it on Trulia.
Explain to your client that there are areas of the internet which the individual honestly has no control over it. If it is listed correctly on the MLS, and syndicated to Realtor.com you are giving it the exposure you have control over. You may also have to call the other listing agent and ask them to remove their old listing from Trulia.