i have a betta in a 10 with a ACmini in it, and he is doing good. no problems with him and the filter at all.
i've noticed that when i first set up my 5 gal, betta would go up to the surface right behind the water flow. i guess there is less current in that area, so he would go over there and rest. is this what yours is doing? try lowering the output just a little bit. the betta was never hurt.
i think its a myth that filters and bettas dont mix. my bettas look happier with a little current, not the overkill of cichlid tanks though.