MBoy

<Window x:Class="DataGridEditbug.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="626">
    <Grid>
        <DataGrid x:Name="customerDG" ItemsSource="{Binding}" AutoGenerateColumns="False" >
            <DataGrid.Columns>
                <DataGridTextColumn Binding="{Binding FirstName}" Header="Last name" Width="0.5*" />
                <DataGridTextColumn IsReadOnly="True" Binding="{Binding LastName}" Header="Last name" Width="0.5*" />
                <DataGridTemplateColumn Width="0.3*" Header="Homework">
                    <DataGridTemplateColumn.CellTemplate>
                        <DataTemplate>
                            <RichTextBox >
                                <FlowDocument>
                                    <Paragraph>
                                        <Run Text="{Binding Homework}"/>
                                    </Paragraph>
                                </FlowDocument>
                            </RichTextBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellTemplate>

                    <DataGridTemplateColumn.CellEditingTemplate>
                        <DataTemplate>
                            <RichTextBox >
                                <FlowDocument>
                                    <Paragraph>
                                        <Run Text="{Binding Homework}"/>
                                    </Paragraph>
                                </FlowDocument>
                            </RichTextBox>
                        </DataTemplate>
                    </DataGridTemplateColumn.CellEditingTemplate>
                </DataGridTemplateColumn>
            </DataGrid.Columns>
        </DataGrid>
    </Grid>
</Window>