Only the thread owner can start a wiki on a thread page, because the wiki would be added to that thread. If the user didn't start the thread, then usually they have to be a mod or admin. There is a new setting in Beta 3 under your forum settings where you can change the strictness of who can create wikis for threads.
If you're not on a thread page and you're in a wiki area, you're not dealing with the thread anymore, so the wiki doesn't care about those permissions and you can create whatever you want.
Once the thread already has a wiki, it shouldn't matter if the user is viewing the wiki view or the thread view, they should be able to see the edit link if they can edit. Both pages call the same function to check permissions.
If the user was referring to a specific thread, please send me a link.