Hi,
Per my understanding, there is an event receiver in your library can detect and delete invalid items, now you want to show a summary of the invalid items to users.
As a suggestion, you can create a custom popup window using JavaScript to display the content you want.
To achieve this, in the event receiver, redirect user to a custom page plus query string after adding items, the information about the invalid items can be added into
the query string. Then in master page, add some JavaScript to extract the query string from URL if user in the current library list view, display a popup window with the invalid items information.
A demo about how to redirect to other page in event receiver:
http://www.c-sharpcorner.com/UploadFile/3a164c/custom-messages-using-event-receiver-in-sharepoint/
About how to extract query string from URL using JavaScript:
http://www.joezimjs.com/javascript/3-ways-to-parse-a-query-string-in-a-url/
How to display a popup window using JavaScript:
http://www.w3schools.com/js/js_popup.asp
Adding JavaScript and CSS to SharePoint
http://techtrainingnotes.blogspot.com/2012/05/adding-javascript-and-css-to-sharepoint.html
A similar thread with some useful posts would be helpful to you:
http://sharepoint.stackexchange.com/questions/44791/how-to-use-sharepoint-notification-from-event-receiver
Thanks
Patrick Liang