Hey Gregoire,
The formula you are looking for is get_field("your_key", "widget_$widget_id" );
where $widget_id
is the unique instance of the widget. This can be difficult to grab dynamically though.
I would look into the global $wp_registered_sidebars
to see if that contains instances of the widget that you are looking for. I do know for a fact that sidebar data is stored in the wp_options
table so you may be able to query that directly to get the unique instantiated widget ID as well.