I would never put a halter over the top if she is out in a paddock, even if it’s only for a few hours, she could get caught on something. Get a better fitting fly veil, or ones that are a stretchy material around the face and the fly mesh over the face
Put head collar on over it
Give another distraction. If she can go out with a horse or give hay etc.
You could duct tape the strap!
Put a halter over the fly mask. Only if your horse goes out in the daytime, not night.
If your horse just gets turned out for a few hours a day you can put the halter over the fly mask