Right now, there is not a way to see logins by individual in model history (our product team does have a request for enhancement logged for this feature).
If you need this information, please file a support ticket (email: firstname.lastname@example.org) with the users and date ranges needed and the support team can work on a report for you.
As per the Administration: Security - Audit page in Anaplan, you should be able to audit if you have access to Anaplan Audit.
You have mentioned "customer" however you have not mentioned what type of customer. There might be one or only a handful people within the company who would have ADMIN role or such kind of roles that would be able to do AUDIT. Therefore you need to check what kind of customer it is.