In my opinion, 谁是她 is not wrong technically nor grammatically.
Context plays an important role. Let's see this context:
B: 是她告诉我的? (assume A has no idea who she is and she is not present at the time)
A: 谁是她？( or 她是谁)
In this case, 谁是她 is applicable.
The above case is very rare because speaker B would make sure speaker A knows who 她 is referring to in order not to be asked to clarify. 她 should either be mentioned in the previous context or at the present(the speaker can point to 她). That's probably the reason why 谁是她 is rarely used practically.
And 谁是李月 or 李月是谁 sounds right because it's very likely that one has mentioned another person's name whist his audiences have no idea who the person is. That's why 谁是李月 could have more chances to be said than 谁是她.
So, my take to this is that 谁是她 isn't technologically wrong but there aren't many circumstances where you could use it. That's why it sounds a bit odd when you mention it alone without context.