What constitutes actionable libel?

This might consitute libel. Libel is the public making of a false statement of fact, that damages a person's (or business's) reputation and/or makes other's less likely to do business with them. While opinions are not actionable--e.g. this woman could say "he and his staff were the worst mechanics ever," since that is an opinion--a statement that your husband and staff drink beer while working is a factual statement. However, bear in mind:

1) The truth is not liable, so if your husband and mechanics do drink beer on the job, there is no remedy;

2) The law doesn't deal in unprovable potential damage; if you can't quantify and prove damage or economic loss in some way, including by the use of some business or financial expers (which can itself be expensive), it may be difficult to bring a cause of action effectively.

You might check with the CPA whether they have a mechanism for disputing or removing complaints and consider that remedy. Otherwise, you might consult with a personal injury attorney, before doing so, check with the agency whether they can point you to any law which makes complaints made to them protected (e.g. not libel). I am not aware of any, but agencies like this are the best informants and generally try to be helpful.