That's the thing though, when she's full aware of the situation it's then her decision. We can make reasonable decisions when we already have knowledge of a person, such as a clingy stalker girl very likely becoming much worse if sleeping with her. We can't be expected to make the judgement that everyone would be hurt by a night of fun though as not everyone is.

Fully disclosure of where people stand allows them to make a decision as an adult. If they make that decision it's on them, if/when that person does display signs of getting in too deep it is then the other party's responsibility to let things go, not before knowing though.