Try putting a large solid ornament in front of the filter to difuse the flow :)

Does it have a flow setting on it? Check the manual. Most filter's flows can be changed. Good luck!

You can also use the plastic canvas stuff used for tank dividers and lids (ask at a craft store) to diffuse it. Just glue one or two layers of it to your filter's outflow, using aquarium sealant for the glue. A small forest of real or fake plants in front of it with the canvas is generally something bettas love their owners for.