The site might be deemed a Business Associate, depending on the specific facts, which we don't fully possess. That's why I recommended the owner seek counsel.
How could this app possibly be considered a business associate to a provider? The provider has no idea it’s even being used, let alone a formal association with the application.
It’s not really that broad. It amounts to medical professionals (doctors, nurses, etc.), insurers, and any systems they use to store or process data. If the medical professional or insurer is not using the app, and the app has not signed a BAA with them, then it’s not covered under HIPAA.
It would be a mistake to assume a SaaS that stores healthcare PII for coordinating healthcare is not covered under HIPAA. An exception should be filed at the very least.
Edit: If no healthcare provider has access then maybe it could skate by. I interpreted "any user making notes to your account" to mean healthcare providers would have access. Even if not, they should still seek legal counsel. And this app is literally promising safety and security of healthcare information.