See bug 18360 comment18 and further. Should we just leave the biblio record if any? Or warn about it, perhaps even checking the number of linked orders, and allow to delete it anyway?
Probably we should allow it since we do so for REST API too. In that case this report is about extending the warning with more information.
Seems reasonable to me to allow deletion of the order line without deleting the biblio record. If someone added a mistaken order line and wants to delete it, it's certainly possible that the biblio record is attached to other orders that are being kept, or may have current items that aren't linked to an order at all. A warn seems like a good balance between not being too restrictive and giving an opportunity to catch "ghost" records (especially if the warn can include information on whether the biblio record has any other linked orders or existing items).