My only advice would be just work on passing in general. Mix it up between open guard and butterfly passes. Then just modify those movements and incorporate them to pass the half butterfly. Drill passing the half butterfly. As long as you react before they put you back in butterfly then there are quite a few places to go.
Also, pin their hips and dont let them get under your hips either. Keeping pressure on their leg with your hand when passing can be important with some of the passes too, since you need to take your weight off their hips to pass sometimes.
If you dont pressure the leg that has the butterfly enough, it can be used to elevate you just enough for them to recover full butterfly, so its always worth thinking about controlling their elevators.