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Thank you for your question. I assume that the reference speaks to dishonesty, not with any sexual/racial connotation. That being the case it would probably be looked upon as a comment that partially diminishes the mutual duty of trust and confidence between employer and employee.
In order for an employee to claim constructive dismissal the trust/confidence between employer/employee must have been breached in such a way so as to completely destroy the trust/confidence. If it had then the employee may have a claim for unfair dismissal. The above comment is not enough in isolation to do this, but note the cumulative effect of incidents can be taken in to account though.
The employee ma consider raising a grievance (if sufficiently aggrieved), the employer will be bound to follow statutory grievance procedure.
Suing for defamation is potentially a possibility, but this would be very expensive (no legal aid) and would seem to be overkill to most reasonable people.
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