If you don't want to install a shitty app you have two options:

1) Hack Whatsapp's servers, break their encryption protocols and eavesdrop on her communication.

2) Clone her sim-card, run it in another phone then start a conversation with each of her contacts you want to be able to spy on (you need to start the conversation otherwise their messages will just go to the other (her) phone instead of duplicating the messages to both. Then hope none of her friends are tech-savy enough to realise the account has been duplicated.

Or... you could install the shitty app. But seeing that she's your ex and wants nothing to do with you, her phone is likely on her persons and the restraining order keeps you away from such persons, I'd say you're shit outta luck.

It's much easier to just hire a hooker and get over her.

You could also hire a PI to follow her around but that costs money.

Have you seen the car Magnum drives, Ferraris don't grow on trees.