Legal? Social media clause in redundancy

confusedmum02

So a friend of mine has been sent this, I’ve redacted the name of the company. I’d just like to know if this is actually okay?! Surely it’s not legal to sack someone on the grounds of what someone else says online? I understand that if the employee themselves was to post something negative online, that would be grounds for dismissal, but I cannot fathom how someone else’s social media post (if it in no way mentions the employee) can be used to sack an employee just because they have some link like a friendship? And can they really say that the employee cannot speak to people who are regular customers, when those regular customers are very likely the employees friends? This just seems extreme and I’d like to know if they are justified or if I’m right and this is not a fair clause? 

The employee has signed a contract which says they cannot slander the business online. It was a clause in the original contract. But the addition of ‘friends etc’ posting is new.