Load content From Include File within plugin

A better solution might be to do it along the lines of

/* In your main plugin file */
if ( ! defined( 'YOUR_PLUGIN_ABSPATH' ) ) {
    define( 'YOUR_PLUGIN_ABSPATH', dirname( __FILE__ ) );
}

/* In any file of the plugin */
if( is_front_page() ) {
    /* adjust path if file.php is in a subfolder */
    require_once ( YOUR_PLUGIN_ABSPATH . '/file.php' );
}

If you still have to deal with full paths, they’re in your file.php.

Additional Option

Also, it might not be the best idea to use a conditional tag (if( is_front_page() )). Alternatively, you could have the plugin generate a shortcode, which outputs the desired markup.

This would have two benefits: For one, it is easily placed and moved within your (front-)page. For another, you can use it anywhere you like, and that without altering code.

/* In your main plugin file */
if ( ! defined( 'YOUR_PLUGIN_ABSPATH' ) ) {
    define( 'YOUR_PLUGIN_ABSPATH', dirname( __FILE__ ) );
}

/* In any file of the plugin */
function your_include( $atts ) {
    /* "path" is a shortcode attribute, you can use it to include several files */
    extract( shortcode_atts(
        array(
            'path' => 'file.php'
        ),
        $atts ) );

    require_once ( YOUR_PLUGIN_ABSPATH . "https://wordpress.stackexchange.com/" . $path );

    /**
    * file.php should be adjusted to save whatever you were echoing in before
    * in a variable (in this example $output), which is returned
    * by the shortcode function
    */

    return $output
}
add_shortcode( 'your-include', 'your_include' );