I am very busy at the moment because of different things, but I have taken a look in the book "Erzherzog Johann, der Brandhofer und seine Hausfrau", where there are a lot of personal letters, infos and pictures, and even if the book is very detailled I could not find anything about a relationship between Nani and the queen of Belgium, Henriette. Through Nanis marriage she was the aunt of Henriette, who was daughter of Joseph Anton of Hungary, brother of Archduke Johann, but I did not find anything about any conmunication or visit of them, I even did not see the name Henriette there. For me it´s not so easy to imagine this close friendship as Henriette lived somewhere else than Johann and Nani, who had residence at the Brandhof.. there is only one picture of the imperial family at Schönbrunn, where Nani is presented to her brother in law, emperor Franz II. and the other Archdukes, but Henriette isn´t on the picture. They could have communicated with another via letters, but I don´t think that there was a close relationship, because the name isn´t mentioned at all in the whole book (or I have very bad eyes
)
It´s quite strange...where have you read about that friendship? Really sorry