Depending on the betta -- it may not mind the current or it may absolutely hate it. The longer the fins, the harder it is on them to swim so keep that in mind. You could easily place a piece of filter sponge over the outflow of the filter to reduce the current, place tall plants in front of the outflow, or fashion a baffle by taking a 20oz coke bottle - cut the top and bottom off and slit it open from top to bottom, and then finally place the bottle over the outflow so that the flow comes out both ends of the bottle which diffuses the current.