PrimeFaces disable validation on cancel button

Use the attribute process="@this" in the Cancel button. This will prevent the whole form being submitted.

Or you can use p:button instead (however this doesn't have the actionListener attribute). See this other Q/A

Use the attribute immediate="true" in your cancel commandButton. This will skip the entire processing of the form, tough, by skipping the Apply Request Values, Process Validations and Update Model Values phases.

<p:commandButton value="Cancel" update=":detailsForm" actionListener="#{userController.onDetailsEditCancel}" immediate="true"/>