If you think she "was" cheating you, you shouldn't be bothered NOW. You say you dont need any spy app.. well sorry to break it to you but NO technique exists which would allow you to just remotely start accessing WhatsApp chats .. Even with many of my mentioned hacks and techniques , SPY Apps (RATs) require a rooted phone to access WhatsApp chats without root access no app can access the chats aswell , at most they can access the SDCard folder of WhatsApp which has images ...
Social Engineering is your best bet to get access to the device, I have answered similar Queries already.