The initial question we should be asking, is what material is it!. Nothing is to be taken as gospel.
Glass or Acrylic?
That will dictate and solidify your responses based on assertive knowledge of the facts.
Soluble means alone, range in increasingly complex chemical properties & scales. To which Im quite sure many of you haven't actually spared a second thought to tests for yourselves - namely to see the results and risk assessments!
We know he's attempted removal on a few occasions, so we narrow a focus. The stain itself is made of the elements that stained it, so lets talk contributes.
If the level cant be breached we delve the abrasive path to solve the issue.