[savedbygrace]

I used to work for a very unintelligent person that said this at least 1/2 dozen times a day.

You used to work for an unintelligent person WHO said this at least 1/2 dozen times a day.

Who, not that, when you are referring to a person.

Also, in what way does 'very' modify 'unintelligent'?

I'm assuming you want to know about these errors.

[Mr Ramsbotham]

Also, in what way does 'very' modify 'unintelligent'?

I wouldn't rate "unintelligent" as a superlative, since there are varying degrees of the condition. A person with an IQ of 70 might be said to be unintelligent, while one with an IQ of 50 could be said to be less unintelligent than one with an IQ of 70. The former, then, could be said to be unintelligent, while the latter could be said to be very unintelligent.