Thank you very much for clarifying. Whether he leaves the organisation is up to him and making threats like that to try and prevent them from working with you is not unlawful – in the end it is up to them to decide whether they would rather have him stay on, or work with you. But what I mean is that someone threatening to stop working with someone if they go into partnership with you is not unlawful, just unethical.
In terms of the slander, it may be a potential case of defamation (this includes libel if it is in written form, or slander if it is in oral form), such claims are rather difficult to pursue. Many people are keen on suing for defamation without having full appreciation of the law or practicalities of doing so. I will try and clarify the position below to give you a better understanding and cover some of these practicalities.
First of all, certain conditions must be met for the statement to be classified as defamatory. These are:
1. There has to be a defamatory statement - the assessment that is often used to establish this is whether the statement tends to lower the claimant in the estimation of right-thinking members of society generally
2. Its publication has caused or is likely to cause serious harm to the reputation of the claimant – this will vary based on what effect it will have but it really has to be something sufficiently serious
3. The statement has been published by the defendant to a third party
4. The claimant must prove that the words complained of were published about them - this should not be an issue if the claimant is named or clearly identified.
Whilst it may be easy to prove that defamation has taken place, the legal process of pursuing such a claim is often complex and prohibitively expensive. A claim must be made in the High Court and will likely require the help of professional defamation lawyers, so the costs will often be high right from the outset, usually in the thousands. There is also no legal aid available for such claims so the complainant must fund these personally.
You must also consider whether the publisher of the statement can potentially defend the claim. For example, this can happen by proving the statement was true or an honest opinion which could have been made based on the available facts.
Instead of starting legal proceedings it would be best to contact the publisher of the comment, advise them that what they have done amounts to defamation and that you will consider pursuing the matter further if they do not retract their statement and issue an apology. A solicitor can also be instructed to write such a letter, although here will be costs involved and it does not always make a huge difference. Either option could prompt the publisher to reconsider their position, which would avoid the need for court action. Of course, if they refuse to comply the option of suing still exists, but consider the above information before going down that route.