It sounds like your client is under the impression that apps behave like websites. You could explore:
- Does the user need to download both apps? Yuck. Going to guess there are a lot of competing products that don't require that.
- What happens if they don't download the Planning app? What shows up in the iframe, a big error message?
- What happens if they download the Planning app, but it's worthless outside of the parent app, the user doesn't understand why it exists, and deletes it?
- Does the Planning app have no navigation or page furniture of its own? Or will it show up to the end user as double navigation, double menus...
- Does the Planning app need no authentication or account creation?
Your client needs to work with APIs, not full apps with UIs.