Yes, it is incorrect.
Notice that the Greek sentence has an article. "Οι εκατό φίλοι της." This means that the speaker refers to one hundred specific people, not just any people that just happen to be one hundred in number.
If that was the case (if the sentence's meaning was generic), the Greek phrase would be "Εκατό φίλοι της" or "Εκατό από τους φίλους της", therefore "One hundred friends of hers" would be an exact and correct translation. ^.^
(Unfortunately this sentence actually has an incorrect translation. A hundred of her friends is wrong, for the reasons I mentioned right above. It's generic, while the Greek sentence is specific.)