Hi Nicolas,
from pictures this doesn't look like a Valjoux 23 - I'd guess it's more a very rare Valjoux 69, which is only 10,5 ''' .
It would allow for an even smaller case, but not always was used in "pure" ladies watches - sometimes in slightly larger cases as well.
Your fiend should definetely open the case back - cal. 69 is one of the finest Valjoux calibers!
Best
Suitbert