Hi @dominic
I doubt the action do_action (‘acf / save_post’) will be of much help to you as the names of your fields (from the WP User Frontend plugin) will not be in the ACF format.
What I would do is research the WP User Frontend plugin docs for an action which is triggered during the saving of the form. Hook into this action and then manually save the ACF data using the update_field function.
You will find documentation about this function over on the docs page.
Thanks
E
