Here’s my take on this.
When it comes to monitoring messages on an iPhone, you generally have two main paths, each with its own trade-offs. The first involves using iCloud. If you have her credentials and she’s backing up to iCloud, certain tools can extract the message data from those backups. The advantage is that you don’t need to install anything directly on her phone. The potential downside is that you only get information from the last backup, so it isn’t always real-time.
The second path is using a dedicated monitoring app. These apps often require a more involved setup but can provide real-time updates on messages, calls, and location. The benefit here is the immediacy of the data. However, these services usually come with a subscription fee and require trusting a third party with sensitive information.