I agree that it is a good lesson for her to learn how to say, "I don't like that. Don't do that to me", but she may be too shy since it is a grown-up (and grown-ups are SUPPOSED to know better than to touch someone's child without the parent's permission!) But, I think this is a bigger problem that one child, one tickle.
If you can preempt the touch, by having your daughter place her hand over her belly, ready to say, "No, don't tickle me." and then you reinforce that by saying to the adult, directly but discreetly, "Please don't tickle my child." that might work. But, he may continue to tickle her on other days or tickle other kids.
So, I would absolutely have a word with the director. It sounds like this person is perhaps part of the staff, no? He needs to know RIGHT NOW, that is never OK to tickle a child with whom you have NO relationship and outside of any context that could be seen as appropriate. Does he tickle all kids? Just the girls?
This is a red flag. You should say something. Sorry this man is making your daughter uncomfortable!