Try finding berghia nudibranch, they work the best but are expensive at times, all you need is a 3-4
He has peppermint shrimp, wouldn't it eat the nudibranch? Or maybe he can remove the shrimp.
I have a filefish in my 32 g tank. It got rid of the aptasia and now eats pellets and frozen food. There is no other coral there only about 10 RBT anemone and it doesn't bother those.
In my 125 g I have a cupper band butterfly. It got rid of aptasia 8 years ago, when I got it. Now it eats frozen, some pellets and it loves to eat the seaweed with the tangs.
I believe if I removed ether these fish the aptasia would come back. Most likely it is still in the tank, where the fish can't reach it.