I’ve seen lots of previous posts which advocate getting as much financial information together as possible (which, to me, would include getting hold of copies of accounts or bank statements).
I’ve not seen anyone specifically say “open his post,” but she’d accessed emails and other electronic information which were personal to him, and you see a LOT of posters recommending that people snoop on emails and texts, WhatsApp messages etc.
The biggest factor in whether you’d be caught up like she was, would be whether your stbex has the money to pursue such an expensive court action. He spent £90k bringing this before the judge and only got 60% of his costs back.
That’s a hell of a lot of money he had to burn for nothing much more than the satisfaction of financially hurting her. Bet their lawyers loved it though.