I got this working so thought I’d share code:
// instance vars.
$uid = $_POST['user_id']; // current user ID
$field_key = "field_5344787df3a9b"; // ACF field name
$post_id = $_POST['post_id']; // Post ID of draw to update
// Get member_entries field
$value = get_field($field_key, $post_id);
$new_value = array();
foreach ( $value as $id => $entry ) {
if ($entry['user_id'] == $uid) continue;
$new_value[$id] = $entry;
}
$status = update_field( $field_key, $new_value, $post_id );