She pretty much uses the Visual editor, and does not use html code. It has been looking good up until yesterday. Now it's fine in IE but not in Safari or FF. When using the W3 validator will the fix option automatically make the corrections, or is that not a good thing to do?
Which versions of Safari and Firefox? Looks fine here in Safari 5 and Firefox 11. It breaks in Firefox 3.6 Mac and, as the Wordpress theme is quite recent, it's possible the designer hasn't bothered to support a version of that age.
The theme is "responsive", meaning that the layout changes when the browser window is at certain widths. Perhaps this is why it seems to look strange.