Ethical reaction to fake academic degree claim. (Unfinished PhD) Let's assume a hypothetical context where someone figures out that a given businessperson's claims to have a PhD from a prestigious university are not true. The person in question was enrolled in a PhD program but they never finished and therefore no degree was awarded. The person is introduced in interviews and presentations as "Dr." and listed in the company's website as having a PhD in a scientific topic from a specific real university.

Provided the evidence is unequivocal, the questions are:

What is the ethical response to this? How to handle it?

Is it fraud?

What are the mechanisms available to denounce this, if pertinent?

(depending on answer to previous question)

I'm personally interested in the case for Europe and UK, but cases for other countries may be of general interest.

This is related to other question about Diploma mills, but my question is about real prestigious universities, real enrolment but no award.

This question does not relate to degrees pending imminent conferral, but to incomplete degrees, falsely advertised as completed.
Obligatory disclaimer: I am not a lawyer and this is not legal advice.

In Germany, using a Dr. title without having earned it is illegal, and it is punished with imprisonment up to one year or with a monetary fine (this is a link to the relevant law in German). 

The mechanism available to denounce this is to report the offending person to the police. Before doing this, I would suggest talking to a lawyer, as there might be consequences if the accusation turns out to be false. If the offending person is working as a medical doctor or some other regulated profession, the corresponding regulating association (for example, in the case of doctors, Bundesärztekammer or Medical Association of Germany) or other fair competition associations might sue the offender on the grounds of unfair competition (this has happened already several times).

The ethical thing to do depends on your particular sense of ethics. What you will do depends on what motivation you have to do it. The other person could sue you for false accusations, and going on a legal fight as a private person is very expensive, so there must be a good motivation. If the offender has been selected over you for some position and you feel that this is due to the fraudulent use of the title, you might as well just inform that particular employer of the situation. Going public is not recommended unless you have talked to a lawyer, as you could land in legal trouble if you make public accusations that turn out to be false.