Private detectives or investigators (PI) are often used by affordable people and the upper class for delicate and discreet issues. A private detective agent has to be licensed and listed in the Central Insitute of private detective agencies(CIPDA). They have to follow the procedural codes of conduct and ethics formulated by the CIPDA. This includes methods, storage and ownership of evidence collected related to a crime. Once licensed the PI can investigate asset ownership, pre-marital,pre-employment background check, insurance fraud, and corporate theft-related issues.
There are strict regulations, for instance even if it's a simple case of a domestic fight in the family, they cannot investigate if a police complaint is pending still, they cannot take on insurance claims of government organizations and archaeological complaints. So, yes they can produce evidence, as long as they are licensed, registered under CIPDA, follow the guidelines in terms of jurisdiction and collection of evidence.
It is the job of a lawyer to present that evidence in the court for the perusal of the judge.
The judge is not bothered for the way you have collected the evidence. They are only interested in its authenticity and relevance with the on going case.
Thoughts exchanged is knowledge gained.
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease