Principles behind non-prosecution:

Consent if the subject has made a clear wish, without any pressure applied. Remember consent is an important implication of Kant’s principle of humanity (or ends), and natural rights are a key feature of natural law.
Motivation if the suspect has shown compassion or attempted to persuade the subject against suicide. Remember love is the key principle of situation ethics and happiness of utilitarian ethics.
Openness if the suspect has informed the police immediately. Openness is linked to the virtue of honesty, again important to Kantians or those arguing from a Religious perspective.
Keir Starmer said: “The policy is now more focused on the motivation of the suspect rather than the characteristics of the victim. The policy does not change the law on assisted suicide. It does not open the door for euthanasia.”