In the situation you've described, to get that error message when running QA/QC on an AbandonedDevice requires both:
1) In the TransformerUnit table, ArcFM Properties, Object Info tab, Esri Rules checkbox is checked, and
2) In the AbandonedDevice_TransformerUnit relationship, ArcFM Properties, Validate Related checkbox is checked.
So to stop seeing that error, pick either one of those checkboxes and uncheck it.
You can find more examples of QA/QC error messages for the many QA/QC checkboxes in Tutorial - ArcFM - Configuring ArcFM QA/QC. You'll need to request access here: The specified item was not found. first in order to see the content (access is granted to all registered users of exchange).