When River heard that it was the season to scare the pants off of folks (for candy, even!) she just couldn't resist. The cyclops mask immediately gave her an idea-- though, Frisk probably wouldn't be getting his mask back after she was done with it. All she had to do was get that eye popped off for a second first, and wait for the sun to go down....
It's amazing what you can do with a well-placed light in the dark.
(Though, I don't think she can actually see where she's going, wearing it like that...)
