I posted also a comment but perhaps this is the right place.
TreeGrid has some limitation on editing but, if I understand well, It's possible editing columns except ExpandedColumn.
I found a problem: each time i call restoreRow another tree icon is added in ExpandedColumn.
After investigation I realized:
1) editRow save in savedRow all the columns (editable and NOT editable). For the ExpandedColumn it's save all the
2) restoreRow restore html from savedRow, and for the ExpandedColumn insert the saved data into a element, so add another icon
I modify editRow (grid.inlinedit.js)