He should be getting an email every time the subscription renews.
You have to ways to deal with this that I know of.
The first is the proper method which is to regain access to the account. This article tells you the various ways you can try to discover your forgotten Apple ID --> If you forgot your Apple Account primary email address or phone number - Apple Support Typically an Apple ID is an email address you have used. If you are currently using a mobile device that is logged into your account, go to Settings and tap your name at the top. Look in Keychain, and in a browser's security settings if you use it to automatically sign into Apple when using a browser.
This article outlines what to do If you forgot your Apple ID password --> If you forgot your Apple Account password - Apple Support
or go directly to this link --> http://iforgot.apple.com/
If you find you can’t sign in, reset your password, or receive verification codes, you can request account recovery (read this support article: How to use account recovery when you can’t reset your Apple Account password - Apple Support ) to regain access to your account. Account recovery might take a few days or longer, depending on the specific account information you can provide to verify your identity. More detail in —> Activation Lockout Issues - Apple Community
If you still need help this page provides country-specific Apple Support contact information ➞ Contact Apple for support and service - Apple Support
Once you do that, this support article explains how to cancel a subscription from Apple on an Apple mobile device, Mac, PC, and online ➔ If you want to cancel a subscription from Apple - Apple Support
The bad approach is to cancel the card (or have them issue you with another one) so the payment defaults. Of course this will then mean you need to change the payment method on any other account using that card. It's also presuming you never want to use that account again.
