Cats hunt because the instinct to do is hard-wired into them, so they will pounce on prey that taste horrible, like shrews or frogs. You can't change the cat but you can help the frog. The best course of action is to add hiding places- anything with plenty of deep cracks to squeeze between is perfect for frogs as a place to escape from cats. Heaps of stones, wood piles, long grass also has potential for hibernation sites, You could cut down the risk of stepping on a frog brought into your bedroom by your cat, by keeping your cat in at dusk when frogs are most active.