Why are you reticent to tell her if she wants to know? This isn't clear to me.
Honestly, IMO, the decision to have access to this information strikes me as being the child's. She has a right to know important information about herself. If she wants to know if she has a talent, she has a right to know, and I think it's very odd that a parent would deny her this, given that she's asking.
It's hard to develop a talent if you don't believe you have one. Her belief that she's average can hinder her in ways you may not have considered. Gifted students often suffer from underachievement, and it's possible that for some of them part of the problem is thinking "those high achievers are smarter than I am," when the high achievers just know how to study.
I know that some people on this forum have said that they wished they didn't know their IQs, but none of you are this girl. Knowledge about something like this isn't necessarily damaging; it's how the grownups deal with it and teach the child to deal with it that causes problems.
You may want to consider if she'll resent you for not being honest with her, as well.
