Definitely tell the manager. He/she needs to know. If you were that manager or an owner of a restaurant, wouldn't you want to know if your employees are doing anything that might cause your customers to go elsewhere?

And as others have said, I would keep the specifics of the situation very vague. The goal is not to get that employee fired but to make sure that ALL employees are aware of the proper hygiene behaviors around food.

I would also add to be ploite and respectful and very specific with what you saw. Don't sound like you are coming across as someone looking for a free ice cream.

Let us know how you handle it and what the manager said.