Chelsea good friend is Ivanka. Maybe that’s why Trump knows Hillary is sick. Ivanka could know what type of medical equipment Chelsea has in the apartment. Friends do visit each other and girls talk about their families.
We don’t know that Chelsea and Ivanka are such “good” friends at all, so far as I have ever seen. There are some references to their being friends before the campaign, but I have never seen anything showing they really have much of a connection.
On the other hand, this summer Chelsea did something nasty trying to embarrass Ivanka into pressuring her father on some issue or other, I forget what. I remember thinking at the time, it was not the sort of thing a real friend would do to a friend at all, especially in public, in the heat of a presidential campaign.