Execute plugin for specific user role(s) only

It’s not a good idea to modify plugins if you can help it, as the plugin may be upated and you will lose your changes and need to redo them.

Fortunately, that is not necessary in this case anyway, as you can the filter active plugins option instead. You will need the plugin slug for this:

add_filter( 'option_active_plugins', 'custom_plugin_load_filter' );
function custom_plugin_load_filter( $plugins ) {
    $user = wp_get_current_user();
    if ( !in_array( 'sales_events', (array) $user->roles ) ) {
        unset( $plugins['my-plugin-slug'] ); // change my-plugin-slug
    }
    return $plugins;
}

Note as this will run on the plugins page also, it will probably prevent anyone without the role from disabling the plugin (as it will appear to be inactive to them.)