He can be doing it via airpods too. One earpiece at his place and one close to her.

Why the fuck is she with this guy? You say their relationship is “healthy”. Spying on your significant others conversations is absolutely not healthy, dude. The fuck.

It's possible he obtained the information through other means. While spyware can exist on iPhones, it's extremely complex and will cost about $500,000 to compile and deploy against a single target. Unless this guy has money lying around to burn on something like this, it's unlikely he compromised the device.

Either way, it's not a healthy thing to do and they should do something about the root cause of his insecurities.