@AC Re: Linkedin?
I think that its a bit ingenious on the part of the author.
In the article he stated:
These obligations also apply to the outgoing employee. In May 2016, the Information Commissioner’s Office successfully prosecuted Mark Lloyd (his real name), an ex-employee of Acorn Waste Management Ltd in Shropshire, for emailing the details of 957 clients to his personal email address along with purchase history and commercially sensitive information prior to taking a role with a rival. In that particular case, the individual in question pleaded guilty and was fined £300, ordered to pay £405.98 costs and a £30 victim surcharge.
Here's the sticky issue. You (Mark Lloyd) have formed a business relationship with the clients.
Taking the names of the clients and their contact information (e.g. phone/email) in itself isn't going to be an issue because the relationship could extend beyond work. This has more to do with what he does with the information.
If Mark attempts to sell the information... he's in hot water. If he passes it on to a new work colleague... he's in hot water. If he sends out a farewell email announcing he's leaving/left the company and provides his contact information... he would not be in any hot water.
What really hurt Mark is that he didn't just take the contact info, but the sales information which is company specific and not in any shape or form personal.
Connecting with clients on LinkedIn, aka your third party, in itself isn't an issue or anything for an employer to fret about, nor something that they could do anything legally about.