You can use the following temporary workaround in order to avoid the problem:
1. Attach OnGridCreated event handler:
    ...
    <ClientSettings>
         <ClientEvents OnGridCreated="onGridCreated" />
    </ClientSettings>
    ...

2. In the event handler configure the group header according to the number of visible columns in the grid:
          function onGridCreated(sender, args) {
              if ($telerik.isIE7) {
                  var visibleCount = sender.get_masterTableView()._getVisibleColumnsCount();
                  $telerik.$(".rgGroupHeader .rgGroupCol + td", sender.get_element()).attr("colspan", visibleCount);
              }
          }