Very personal two cents worth, I'm getting a very bad vibe off this psych. "I've never seen it so it can't be true" is not the most professional reaction.
I couldn't agree more that it's important to understand how the giftedness impacts the disabilities - it will change how you diagnose, how you remediate, and how you assess progress. With an HG+ child, it may change those things considerably, in ways that are hard to imagine if you haven't seen it before - certainly enough that I would no confidence this particular psych would know how to do it. To play my broken record, you truly cannot overestimate how much these kids can compensate and mask and find alternate ways of doing things - and completely hide how hard they struggle and what it costs them to look like they can do it just like everyone else.
As for the testing, unless she has a better concern than "I don't believe these results" (such as they were done on a very young child), I can't see rejecting and repeating respected testing performed by a respected professional. If it is remotely feasible, I can't say strongly enough how important it is to work with someone who really understands the whole interconnected 2E picture, and not just one isolated piece of it.
