We have a "rogue" accounting employee who has made comments about other employees' excessive health costs in front of other employees. Is this a reportable HIPAA violation?

Probably not. Unless the employee discussed the nature of the treatment being given, its probably not going run afoul of HIPAA. However, it is a serious breach of business ethics and this employee should be terminated.

Do you work for a healthcare company ? HIPPA applies to health care providers.

Are they giving details about the actual care, not that iti merely excessive costs.

The only way this might be a HIPAA violation is because medical insurance info needs to be kep confidential. So the question becomes how did this employee get the info.

Is it safe to assume that he got this info because he does the accounting? That is a breach of business ethics but a HIPA violation would be if someone talks about the persons care. If for example someone works at a hospital, you cannot even tell someone that a person is in the hospital with very very few exceptions and under extreme circumstances which have to be approved by a super or a manager. You definitely cannot talk about why a patient is being treated and/or what for. Grounds for immediate dismissal. You can let H.R. know what you are hearing but I would try to keep it anonymous. Maybe a letter or something. You don't need someone trying to be vindictive or get you fired.