Yes, that is the solution in this case.
I will explain what we can assist with:
1. Create a user field, this can be a simple single line
2. It will hold the User ID of the user that approved the current user
3. To update this field, you can either do so manually in each User's Profile in the backend, or thru a CRED Form, or with custom code.
Since you rely on a 3rd party to allow the user (Hence, to update this user), you need to hook in their code.
You need to know when a user is updated. They will have a function that does this that you maybe even can filter (this is a question to direct to those developers).
Once you have that, you can hook in with a simple update_user_meta(), where you update the above-created user field with the value (user id) you need.
https://codex.wordpress.org/Function_Reference/update_user_meta
The only special here is that the User Field created by Types will have a slug (metakey) prefixed by wpcf-
This means, when you update the user field with the code, you need to do something like this:
//you need to find out how you can get the user ID that is approving another user. Direct this question to the 3rd party developers, as this is not made with Toolset
$creator_id = '2';
//this is our Field where we will save the user id (creator) against the approved user
$creator_field = 'wpcf-your_slug_here';
//again here you need to know how to get the "approved user" ID, ask this question to the developers who develop this system
$approved_user = '3';
//Now we update the field with all data
update_user_meta( $approved_user, $creator_field, $creator_id);
Now, you could create a user View, list all users, and add a Query Filter for:
Select items with field:
your_field_slug is a string equal to equal to VIEW_PARAM(your_param)
Then, you can insert this view in a page by passing the user ID in the "your_param" attribute.
This would then for example display all users where the creator is of the same ID.
But as you see, our assistance here can be limited only
