I would provide notice to the parent. IMO the parents is the root of the problem. A toddler doesn't have to be taught to hit via the actions of another. It is just a natural reaction to frustration that they can't vent any other way. The problem is that if that behavior is not corrected, an aggressive child is created.

Since you have spoken with the parent about this before and no change has been made, it is pretty clear to me that no changes are going to be. I have learned that I cannot create skills in a parent that they do not have, nor are interested in obtaining.

I think though your dedication to being proactive with the situation and diplomatic with the parent is inspiring. Let us know how it goes.