The rules of iTunes connect are only really available when you sign in to the service, and agree to the terms. I am not currently a developer so I don't know even if the details are protected by an NDA.
I don't have a link to a good press article covering the current state of affairs, but my understanding and gross oversimplification of the rules are:
- It's OK to enable extra content if the iOS app does nothing to link to, process, enroll or advertise the paid portion of the service or app.
- It's always OK to handle purchases through in-app purchase where Apple will take their cut of the sale in return for providing the accounting, customer service, billing, returns, server hardware, software and bandwidth for distribution of your app and the handling of the paid content sales. It's probably very not OK to sell in the app and not use Apple for the payment.
- Newsstand apps are different and I'll let someone who knows edit in how a subscription might be different than a one time sale, but I would be surprised if it was overall different than the normal in-app purchase policy.
If your work for your client is to advise them, you'll really want to just spend the $99 and join and have your team participate in the developer program NDA / restricted chats and or just read the terms of iTunes Connect. If not, it's their problem and make sure your contract with them is clear as to under whom all responsibility of knowing and adhering to the Terms and Conditions fall.