Question

I had not been / wouldn't have been his friend if i'd known he was so horrible

Answer 1

I wouldn’t have been his friend...

Answer 2

The sentence "I had not been his friend if I'd known he was so horrible" suggests that you were his friend in the past, but if you had known about his horrible qualities at that time, you would not have become his friend.

On the other hand, "I wouldn't have been his friend if I'd known he was so horrible" implies that you are currently his friend, but if you had known about his negative traits earlier, you wouldn't have become his friend in the first place.

Both sentences convey a similar meaning, but the first one refers to a past friendship while the second one refers to the present situation.