I think theres two answers to this
The first is practical. Yes there are ways of reading your messages if you have access to someone's accounts. I would hazard a guess that he has access to her apple account, and whatsapp and i signed on as her on another device. You can have a computer version of someones WhatsApp for example and if you log in as someone on google you can see their searchhistory. Find my iPhone would also allow tracking, as well as any number of apps. The best way to start would be factory resetting her phone, checking where else her Google, apple, what's app, facebook accounts etc are logged in. (Or setting up new ones!)
HOWEVER THIS IS LIKELY TO NOT BE SAFE. Especially if she is still living with him, then the priority is her safety.
Leaving is a really risky time for a woman, and he will know she has suddenly reset all her devices. I wouldnt personally do it until I was in a position of safety and leaving the house. Personally at that point id factory reset the phone and just leave it in the house to be safe.
Knowing she is being tracked is an incredibly powerful thing, and allows her to make sure she isn't giving him dangerous info eg info on rental agencies, searches for dv support by accident. She needs to assume that he can see every message, call, google and email on that phone. She can also lay deliberately misleading trails so he doesnt know where shes going to live after.
Having access to a second phone means she has a seperate way to get help, Google to her hearts content etc and she needs to be careful about protecting that and not revealing it. It might be her only unmonitored communication.
My advice is to not use that clean phone at home, or do anything to link it eg join WiFi networks, cars bluetooth and joining any of her existing accounts. Keep it completely clean
Some services like facebook, banks, Amazon will send an alert if theres a log in on a new device for example alerting him to her new phone, and if she continues using an amazon account he can access after she's moved he will be able to see her new address.