~Slander is spoken, libel is written, so he could sue for lible, but not libel.
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These are two defences to libel that would be available to the sender of the email:
1. Justification
The defence of justification, or truth, is a complete defence to a libel action. The onus is on the defendant to prove that the allegations are true.
Whilst the defendant does not, necessarily, have to prove the truth of each and every fact, he does have to prove the truth of the substance of the allegation, the defamatory 'sting'.
2. Fair comment
Expressions of opinion, based upon true facts, made in good faith and without malice on a matter of public interest may be protected. Note the distinction between fact and opinion.
The facts must be proved to be true and the opinion, the subject of the fair comment defence, must be honest. Therefore the facts upon which the comment is based must be correct. It does not matter if the view expressed is extreme, so long as it is an honestly held view and not malicious. For this purpose, 'malice' may be established if it can be proven that the commentator did not genuinely hold the view he expressed.
It is very very expensive to pursue a claim for libel or slander through the courts, and as the remedy is financial compensation it is totally pointless if the defendant has no money or assets.
In the current climate your relative may find it difficult claiming that cancellations are due to the email - it could simply be because the parents' can't afford to continue at this time. However, damage is presumed in libel cases so your relative wouldn't have to prove this.
I would advise your relative to compose one very well thought out email to send to the recipients of the original email. In it, he can reiterate his qualifications and give a contact where the qualifications can be substantiatedl and