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He is a friend of my father = He is one of my father's friends, which means "your father has many friends, he is one of them(many friends)"
It does not say whether he is a good friend or bad.
Not exactly. "He is a friend of my father" doesn't mean that your father has many friends, but it does mean that the person is one of your father's friends. As for "bad friend", that expression doesn't exist in my neck of the woods (where I live). Don't consider "good friend" the opposite of "bad friend". Instead, "good friend" is more like "close friend".
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