If she ordered the test yesterday. It would come today. How did she manage the test and get the results, today?
Something doesn't add up. I wouldn't drop a neighbour, who I have had no contact with a note to say I was covid positive. That seems quite unlikely.
She then got symptoms quite quickly after, finding out they had it.
She then catches covid, despite having no contact with anyone. Not impossible but, again, very unlikely.
Then manages to get a home test done and results, quicker than everyone else. Again, very unlikely.
All these things have very very low odds of happening, on their own. But all these unlikely events happened to your dd, leading her to cancel a visit that was planned.
She doesn't see anyone for weeks at a time now doesn't want you to visit? I would be concerned for her and/or presume she isn't telling you the whole truth.