@WBYeats,
Your teacher was wrong. Whom is the objective pronoun. The sentence "She did not meet whom he loves" can be said to be awkward, but it is not incorrect. A useful way to determine whether or not one should use whom is to substitute the third person, singular pronoun. "He loves Sarah. She did not meet
her." Therefore, if the person is unspecified, "whom" is the correct way to refer to that person. "He struck Al. She did not see
him." Therefore, one would use "whom."