This is working for me, it updates the last modified author when editing from the front end.
Just add this to your functions file.
function user_acf_save_post( $post_id )
{
update_post_meta( $post_id, '_edit_last', get_current_user_id() );//update last modified by
}
// run after save post
add_action('save_post', 'user_acf_save_post', 10);