<Grid>
    <Grid.ColumnDefinitions>
        <ColumnDefinition/>
        <ColumnDefinition/>
    </Grid.ColumnDefinitions>
    <Grid.RowDefinitions>
        <RowDefinition Height="Auto" />
        <RowDefinition />
    </Grid.RowDefinitions>
    <StackPanel>
        <RadioButton x:Name="RadioButton" IsEnabled="False" />
    </StackPanel>
    <TextBox Grid.Row="0" Grid.Column="1" x:Name="TextBox1" TextChanged="TextBox1_TextChanged" />
</Grid>

private void TextBox1_TextChanged(object sender, TextChangedEventArgs e) {
    TextBox textBox = (TextBox) sender;
    if (textBox.Text.Length > 0) {
        RadioButton.IsEnabled = true;
    } else {
        RadioButton.IsEnabled = false;
    }
}