You all have made excellent points. When I meet up with my friend (maybe next week) I know how to approach this!
I'm not mourning the hen night still...but I am wary of inviting J anywhere to anything that involves alcohol. It doesn't mean I'm holding any kind of grudge, just that I'm being more safe than sorry. It wasn't just her drinking that created the nightmare. It was her reprehensible behaviour that went with her uncontrollable urge to get wasted on what was intended to be a great meal and a good evening of fun.
She has since admitted to me that her drink was not spiked, she was just scared and knew I was angry with her. Basically she lied for her own ends. I was relieved she wasn't lying to me any more. But it doesn't alter the fact she kept up that lie for months before telling me what really happened, which we all saw anyway!