I always like to write about former cases and some of the issues I have ran into in the past in hopes that I can answer questions for someone later on down the line. So I had a client that ran a business with a fellow family member and lost a lot of records in supposedly a robbery that had occurred. She had a lot of former employee's she needed to draw up tax documents on that she only had on for a short while but no longer had their tax information. She did have a list of names though and wondered if she could get the socials of her former employee's in order to fill out these tax forms.
The short answer : Not without the owners permission.
A social security number is federally protected information used to identify a person. This number cannot be given out by any third party without the owner's permission (I always prefer written just to be on the safe side). This is to protect the owner from potential identity theft.
Now this woman could have been a legitimate employer that really only needed the social security numbers for actual tax purposes. In which I offered a solution for her by providing contact information for each of her former employee's so she could gather that information herself. She was not interested in this route, which was a red flag for me. It is not uncommon for someone to lie about who they are to a private investigator in order to convince them to obtain information for them illegally. However, a good private investigator will not only have a great moral and ethical guideline but also value the idea of keeping their professional license.
It's also good for a private investigator to not only investigate on behalf of the client, but also check the client as well. Just to protect themselves. It's a PI's job to not only operate on the best interests of the client, but to ensure everyone stays within the legal realms.