NickBanglo:

Maybe she has already done this, but is there not a reasonable step of saying "this is truly intolerable, and I do not believe you have understood how seriously I mean it when I tell you to stop. The next time you tickle me, our relationship will be over."

Unreasonable?

That's the nicest way to do it, unless of course, she thinks he will try to prevent her from leaving. If so, she needs to pick up and go when he's not home.