Heare is the screen shot of the error that would be displayed.
In order to resolve this the developer needs to review each and every artifact that is loaded for that webpage. Here again Firefox with the Firebug plugin can come to the rescue. Using Firebug one can see all the artifacts that are loaded and anything that is loaded with https can be found. Subsequently all these url need to be changed to use https, that would resolve the issue.
However the client browser can also change a flag in thier IE security setting to disable this warning if that option is applicable. Here are the steps.
1) Launch Internet Explorer.
2) Click on the Tools Menu.
3) Go to on Internet Options.
4) Go to the Security Tab.
5) Click on the Custom Level button.
6) Under the Miscellaneous section look for the setting "Display Mixed Content".
7) Click on Disable for Display Mixed Content instead of Prompt.
8) Click on OK twice.
9) Finally close and reopen the Internet Explorer
10) Try out the same web page that was displaying the warning message, the warning message should not appear any more.
The following video show how to do the following for easy reference.