Let your data column have a cell style with DataTrigger which sets the Tooltip depending on the value:

<telerik:GridViewDataColumn.CellStyle>
    <Style TargetType="telerik:GridViewCell">
        <Style.Triggers>
            <DataTrigger Binding="{Binding StadiumCapacity}" Value="42055">
                <Setter Property="ToolTip" Value="THIS IS TOO SMALL" />
                <Setter Property="Background" Value="Red" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</telerik:GridViewDataColumn.CellStyle>

Initially, when the grid loads - the tooltips are set correctly. But if you edit a cell and change its value to 42055 then the background is set to Red (as expected), but the tooltip does not update.