IAAL and I’ve come across this recently.
In our case, a witness simply refused to answer questions after voluntarily giving a statement. Technically, any non disparaging statements clause DOES extend to giving evidence VOLUNTARILY in an ET. So they have a choice to either break the terms of the SA, or refuse to give evidence/answer specific questions.
The exception is if they are called as a witness by a witness order - in which case, they have to answer questions, even if the answers are negative, and the employer cannot then sue them or take action on the back of it.
So what the employer needs to do is make a witness order application compelling him to attend, as that would mean evidence can be given freely - because a witness who attends but refuses to answer questions does not go down well! Otherwise, if he attends voluntarily, he may be forced to breach the SA anyway, which isn’t exactly an ideal scenario.
It’s also worth checking if the payment in the SA was conditional upon him giving evidence in future - as opposed to a warranty, which might make it less likely they’d be entitled to recover the entire SA payment from him (they could still technically sue for damages, but it wouldn’t be a fundamental breach of contract).
Id say he probably needs his own legal advice (but my gut feeling is that it wouldn’t be worth the employer suing him for the £1k they paid him even if he did breach, as their legal fees would far outweigh that!)
If he hasn’t signed the SA yet, he could ask for it to be removed. And they might not even want him as a witness anyway. But if he’s a named respondent, that would be different, as he could end up personally liable for compensation so needs to be able to freely slag off the employer!