You can check if the Whisper HOB has a flow adjuster, a lot of them do actually, so you can turn the flow down if need be. Floating plants will also help, as will anything else breaking up part of the flow of the water. Best of luck, it does sound like a nice environment for a betta! FYI, crowntails, like females, have less delicate fins, that are less prone to shredding.
Just be aware, if you do get nervous and shut the filter off, make sure you do religious water changes and monitor the parameters carefully.
Is the tank cycled yet? If not i'd recommend reading up on that.