I'm wondering what about the situation is getting to you the most. That might help you figure out how to approach it. For instance, if it is just that her interest in the topic exceeds yours, I think that it is fine to tell her that.
If, on the other hand, the major issue is that you think that she is wrong in her assessement of her child (it sounds like that is part of the problem for you), there is no nice way to say that and referring her to a tester is a reasonable place to start.
I've run into a parent in the past whose assessment of her children (who admitedly are GT ided) is somewhat inflated IMHO. In thinking about it, what really made that relationship and those conversations hard for me was not solely my pet peeve about overidentification but that there was an inherent competition going in a lot of the conversations along the lines of 'my dc is reading at x level, what level is your dc reading at?'
I think that, as you said, when you have a child who is a bit of a different animal than the typical GT ided kid in your area, it is hard to have constant conversations in which you genuinely discuss different things and in your heart and words present and feel that they are the same.
Is this mom implicitly asking you to assume that you two are dealing with similar needs? Is that a problem for you if so?
