I wouldn't do it. Though frogs have been killed by bettas, I would be more worried about the betta. ADF's are nearly blind and can be very aggressive feeders. One of my frogs attacks my hand almost daily when I reach in to put food in their dish. Others are much slower coming around to their dish, and if a betta was around, there wouldn't be anything left by the time they got there, and I'd eventually have a sick betta from overeating.