I might consider reaching out to former colleagues at company B to see if they know this person and what they think of them. I think this is pretty morally dubious, but is it actually illegal?
Well I don't know about legality the UK, but why on earth would it be morally dubious?
I always do this for any potential hire that I'm asked about.
You want to know what kind of worker they are. Hiring is expensive, firing is even more expensive and can be traumatic to the people involved.
I would like to point out some things:
- You only ask people at the other company that you know to be good workers.
- You ask all of them that you can because person x at the old company might say they're bad because those two people don't like each other... and you don't care about person x not liking them... person x doesn't work at your company.
- You don't tell the person you're asking that they have applied for a job at your company.
- You don't assume that the people are correct about that person. It is just additional information.
Along the lines of the last point I was asked once about a person who was terminated. I said that I would give her a great reference if anyone asked me about her. This stunned person x - to which I replied, "Well at her termination there was a lot of bad stuff going on in her life + there was a lot of baggage here which made things worse. I doubt she'd act like that at a new company... I think she'd do a good job... just like the work she did the first few years here." (She did a great job for several years before things went bad.)