To accomplish this task, I suggest that you use the TableView.ColumnHeaderContentStyle property. In this style, you can bind the TextBlock.FontHeight
attached property to the GridColumn.ReadOnly
property (via converter):
<dxg:TableView.ColumnHeaderContentStyle>
<Style TargetType="dxg:HeaderContentControl">
<Setter Property="TextBlock.FontHeight" Value="{Binding ReadOnly, Converter={StaticResource YourReadOnlyToFontHeightConverter}}"/>
</Style>
</dxg:TableView.ColumnHeaderContentStyle>