Interesting.
I have all Muriel Spark's novels and stories except that one and it is annoying not to be able to double check if it said robe or dressing-gown in the original.
I found another reference of "
Art nouveau maiden in long flowing robes. "
http://www.art-nouveau.co.uk/misc6.html
In Paul's reference, Mrs. Fuller, née Sheldon, has a period behind her marital status indicator, we assume she is American. I assume also that she was not at the fete in her bathrobe (dressing gown) as the Spark character (in the quote) was in bed in hers. If that is indeed the passage Sam is referring to.
My theory was that in Sam's copy of M. Spark's book, the words "Liberty dressing gown" was Americanized into "Liberty robe" meaning bathrobe or perhaps more properly Art nouveau.That was how I found the reference to
Seduction, Simulacra and the Feminine: spectacles and images in Muriel Spark's The Public Image and it does say "Liberty dressing gown".
In another report in that article it says someone, "...wore an Empire robe of white satin..."
This sort of speculation is a translator's delight--or nightmare.