I've been keeping ADFs in with my daughters betta for about a year. I had the exact same thing happen after a cleaning but I know that I accidentally nailed her with the gravel vacuum and that is what I attributed the death to.

If I were you I would check the ammonia level, nitrate, nitrite first, make sure the levels are good. Also give the remaining frog a good inspection. Check for odd looking marks, funky mouth, eyes etc... If all is well I would add 3 more as they do much better in groups.

I also highly suggest feeding redworms. The betta and frogs will both eat them and a little frozen pack will easily last a month.