No, the user won't see that you've deleted the message you've sent using Viber, but that message is now stored on the Viber's servers waiting to be delivered when the user comes online. Unfortunately, there is no tool or a way to delete the message from the Viber's servers. As soon as the user comes online, the message will appear to be sent by you. The two check marks will appear next to the message saying Delivered/Seen.
