the swfupload plugin could definitely be an issue.
also the CKEditor version Power CMS Builder does not include IE9 updates. you can upgrade the CKEditor files to use CKEditor 3.6.2:
the CKEditor install page has instructions for upgrading an existing version:
These are the same steps to upgrade the Power CMS CKEditor.
If the CKEditor upgrade does not work, you may need to have the client use IE8 compatibility mode in IE9. Do this by clicking the F12 key to open the developer console. In the developer console, you can select the browsing engine to use.