<TextBlock Text="{Binding Path=Name}" FontSize="14"  >
    <TextBlock.Style>
        <Style TargetType="{x:Type TextBlock}">
            <Style.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TextBlock}}, Path=PlacementTarget.SelectedItem.Id}" Value="1">
                    <Setter Property="FontWeight" Value="Normal" />
                </DataTrigger>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type TextBlock}}, Path=PlacementTarget.SelectedItem.Id}" Value="0">
                    <Setter Property="FontWeight" Value="Bold" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </TextBlock.Style>
</TextBlock>

PlacementTarget.SelectedItem.Id

 public interface IEntry
    {
        int Id { get; set; }
        string Name { get; set; }
   }

<TreeView Name="GroupView" Grid.Row="0" Grid.Column="0" ItemsSource="{Binding Path=GroupList}">
<TreeView.Resources>
    <HierarchicalDataTemplate DataType="{x:Type wsl:GroupActivity}" ItemsSource="{Binding Entries}">
        <StackPanel Orientation="Horizontal">
            <Image Width="16" Height="16" Margin="3,0" Source="Images\softwared_16_16.png" />
            <TextBlock Text="{Binding Path=Name}" FontSize="14" />
        </StackPanel>
    </HierarchicalDataTemplate>
    <DataTemplate DataType="{x:Type wsl:EntryActivity}" >
        <StackPanel Orientation="Horizontal">
            <TextBlock Name="txtName" Text="{Binding Path=Name}" FontSize="14" FontWeight="Normal"  >
                <TextBlock.Style>
                    <Style TargetType="{x:Type TextBlock}">
                        <Style.Triggers>
                            <DataTrigger Binding="{Binding ElementName=GroupView, Path=SelectedItem.Id}" Value="1">
                                <Setter Property="FontWeight" Value="Bold" />
                                <Setter Property="Background" Value="Red"/>
                            </DataTrigger>
                        </Style.Triggers>
                    </Style>
                </TextBlock.Style>
            </TextBlock>
        </StackPanel>
    </DataTemplate>
</TreeView.Resources>
</TreeView>