As a lawyer who doesn’t practice in your jurisdiction and doesn’t specialize in defamation law, I can’t represent you or give you legal advice. However, I can provide some general discussion on the broad themes of defamation law, but if you have specific questions, please contact a lawyer in your area.
Defamation law is tricky and quirky, and it is fairly rare to win a defamation case, and rarer still to win substantial damages.
Putting aside the differences between libel and slander (which can be significant), many jurisdictions require that the plaintiff plead the defamatory language in haec verba, meaning in the exact words. If you cannot find the exact defamatory words used, you may be unable to set out a viable legal complaint.
For a comment to be defamatory, it must be shown to be a factual statement that is false. Comments that are opinions are, in most cases, not defamatory because they are statements of someone’s belief, not a statement purporting to be a true factual statement.
The allegedly defamatory words must legally be considered to be defamatory as a matter of law in the context which they were uttered. Certain statements regarding loathsome conduct are defamatory per se, meaning defamatory in themselves, but most others must be analyzed to see if they would actually be legally sufficient to injure someone’s reputation in the context they were given.
Most jurisdictions also provide a “fair commentary” qualified privilege for various types of comments made and opinions expressed in good faith in the course of some business relationships. Commenting to a business on the conduct of a its employee in the course of contracted work may well fall into this privilege in many jurisdictions.
Even if defamation can be shown, depending on the type of defamation, there is often a requirement to demonstrate actual damages resulting from the defamatory comment.
Finally, there is the extra-legal question of whether it is wise to sue one’s employer’s customer.
All in all, there would likely be significant (and potentially insurmountable) hurdles to overcome to win a defamation case based on a customer’s comment to one’s employer.