Knowledge about Shar may be sparse in this area, at least without a sufficient Religion check. It could be that Shar has been operating in the shadows so long that people don't know a lot. Certainly not more than rumor and hearsay.
Sure, I'll agree with you that a religion check would be appropriate. Which is my main point: the player should have some way of being informed about Shar. If Tav should know about Shar, then the player definitely should.
And even if (your) Tav shouldn't know about Shar, the player should still be given information about Shar through journals/pop-up text/dialogue, so they can have the choice of how to roleplay Tav.
