As much as I can see where your friend is coming from and I can see why they would want the mask back, I’m definitely on the park’s side in this instance.
I’m not sure about Thorpe specifically, as I’ve never looked at the signs that closely, but when I’ve been to parks in America, one thing that they have always clearly and explicitly banned alongside weapons and such is masks. That’s always made very clear at the entrances in those parks, and they have an absolute no tolerance policy, so if you turn up with a mask, you do run the risk of getting it confiscated and permanently losing it.
As much as your friend may have had perfectly innocent intentions, masks can be used in a nefarious manner, and a question of “Are you going to wear it?” is an easy one to wriggle out of even if you do intend to wear it, so I can see why the park are taking no chances.
I can feel your friend’s pain at having a mask they spent hours crafting confiscated and potentially never returned to them, and I do feel sorry for them if the mask is not returned. The park could perhaps have been a touch more flexible in this regard.
With that being said, a mask is a banned item in the exact same vein as weapons, toxic substances and the like. If someone waltzed up to Thorpe Park’s entrance with a knife and had it confiscated, would people be so open to the idea of the owner being given it back? I know that’s not a perfect analogy, but from where the park is standing, masks are forbidden items exactly the same as knives are, and believe it or not, there are similarly innocent reasons why someone could be carrying a knife about their person (for instance, a lot of people carry Swiss Army Knives and such about with them day to day).