One group of native speakers might use to there, and another might use for, and a third group might use these prepositions interchangeably.
Who is it visible to? and Who is it visible for? could both be paraphrased as "Who can see it?"
Some speakers would recognize this distinction: visible to is a statement of the bald fact that something can be seen and visible for is a statement that the visibility is intentional. They were meant to be able to see it.
Some speakers would not recognize that distinction.
Since not all speakers use these prepositions in exactly the same manner, there can be some doubt about whether the visibility is intentional and you will have to rely upon context to make that determination.