Hello,

I think it would return error only in condition if rowObject is not the current row that has pending changes.
Instead to do validation in that event, I try to use the same validation in OnAcceptAllChanges event.

function WebGrid1_OnAcceptAllChanges(controlId) 
{
            var WebGrid1 = ISGetObject(controlId);
            var answer = confirm("Update data ?");
            if (answer) 
            {
                return true;
            }
            else 
            {
                WebGrid1.GetChanges()[0].Row.UndoChanges();                 
                return false;
            }
}

To undo specific row changes, you would need to know in what index array that the row changes collection. You can use WebGrid1.GetChanges().length to know how many row changes.
Hope this helps.

Regards,
Handy