My 30+ years experience as a Manager has taught me not to act hastily on a complaint or info from just one person.

Very often if you don’t get the full picture by listening to the party, you’ll get pie in your face.
People don’t want to be caught telling an outright blatant lie.
So very often they tell half- truths, consciously or unconsciously, designed to lead you to conclusions that validate or justify their actions, behavior or feelings.

Half-truths equate to half- picture; not a complete picture.
If you cannot get the other side of the story, then you must probe carefully through asking the right questions to pry a more complete story out from the person.