I’m using an animation template. Animation templates don’t know what sequences are being used in advance so it can’t know such an event will exist.
I however found a solution. I create a new BP that parents AnimNotify. In that I override Received_Notify and call GetOwner on the MeshComponent. Next I call my interface function DoneAttacking. This sets the IsAttacking state on the player back to false, which the anim bp picks up in its animation instance update tick.