the open space thing is cruel and cowardly.Dolfette may be right; knowing this could help her realise that she has to work on her personality & expectations in the future.
But do it in an open space. Getting the gist of her temperament, it may not turn out pretty...
if i lied someone i would not risk their tears, their distress and humiliation in a public place for an audience to witness.
far kinder, if you want to avoid conflict, to put it in a letter and at least give her the chance to keep her dignity.