Since you have no firsthand knowledge of impropriety, not only do you have no obligation to report but it would be unjust to say anything based on secondhand information. However reliable you believe your source to be, it is possible that the person has misconstrued the situation and that you would be passing along that person’s error. If that person has firsthand knowledge of sexual impropriety by a professor toward a student, then it is that person’s responsibility to take action. If this person does not have firsthand knowledge, then he has no business passing on gossip to you.
Even if he did have firsthand knowledge, there is no reason he should be telling you about this; only those concerned with the issue, such as the school’s administrators, should have been told. In short, you’ve been given nothing more than as-yet unfounded allegations and need only ignore them and refuse to discuss them further with your source.
