Event Receiver Popup msg without cancelling the Item

Hello,

   when user updating an item, I need to show a warning message when there is no item for a particular category. But I have to allow user action to continue. I have custom error page which is working fine when I use  properties.Cancel = true; but it is cancelling the action.

I tried properties.Status = SPEventReceiverStatus.CancelWithRedirectUrl; this also cancels the action

In simple, I don't want to cancel but need popup message.

Thanks in advance.