@RemyZee
What you should do
now is clear:
Tell her that you've spoken to people (here) who use Facebook routinely.
They've told you it's
impossible for you to have accessed her Facebook accidently, and that the
only explanation is that she had borrowed your computer, used Facebook, and left it logged in by mistake.
Stop there: don't ask for an explanation.
IMPORTANT:
That's
all you should say. Don't feed her
any additional information whatsoever.
For example
Do not explicitly ask her to explain herself
Do not add in "it's ok that you borrowed my computer without asking" or anything remotely like that.
Do not add in "you should not have used my computer without asking."
Do not indicate that there may be an innocent explanation on her part
Do not mention that weird advertisement or any of that other stuff
This is basic defense. It's also simple to execute. But you
must stay on point.
Don't react to questions about anything that could be on the "Do Not" list above
If she's the problem, and you provide unnecessary information, she
will misuse it. You must defend proactively against that risk until you're 100% certain she is
not a problem.
Follow up:
What she does next is unpredictable, but what
you do isn't so complicated.
Keep focusing on
her explaining to
you how you could have become logged in to her Facebook account.
Don't answer (literally ignore) questions about: your possible motivation, possible hacking, that weird advertisement, and any of the other weird stuff.
Stay on point: It's not possible for you to have accidently logged in to
her Facebook account. The only explanation is that she made this possible. (Perhaps innocently. But
she has to say that, not you).
Ending the discussion:
* Stop if you get a valid explanation and an apology.
* Stop if it starts "looping" (she tries deflecting to avoid explaining why this strange thing happened)
* Stop if she won't talk to you.
In all exit cases, stay calm and polite. Leave and take a walk or a coffee at a cafe or something like that. Take an hour (clears most of the the adrenaline, and reflection will help).
Ask her along in the first case, and maybe upgrade the location. But you'll
need the time - you can "think" adrenaline into your body, but you can't accelerate getting it out again.