Thesis Existentialism can ground an ethics, and Beauvoir showed how
Where it stands
The standard charge is simple: if there are no given values, then anything goes. Sartre promised an ethics at the end of Being and Nothingness and never published one; the Notebooks for an Ethics came out only after his death.
Beauvoir did write one. In The Ethics of Ambiguity (1947) she argues that to will oneself free is also to will others free, because my projects need a world of other free people to have any meaning. She then examines failed attitudes (the sub-man, the serious man, the nihilist, the adventurer, the passionate man) and shows how each betrays freedom.
My thesis: this gives real constraints. It condemns oppression, for instance, not as a matter of taste but as a contradiction of freedom itself. Who disagrees?