A group of friends is being stalked by an unknown person (or bot?).
Everything started with person A. A was sent authentic screenshots of several of his WhatsApp chats. Screenshots were sometimes sent from an unknown number in WhatsApp, sometimes using other Social Media.
Later, A met B and they started a conversation on WhatsApp. Soon B also received screenshots of several of her chats.
B just clarified neither A or she receives actual screenshots. The messages are authentic, they also contain authentic pictures sometimes. But the look is different, as if a screenshot was made in another app. It's quite different from WhatsApp, for example, the background of every message is plain white. Also, the pictures only show the messages, nothing else. B also told me that some messages directly address her. These are not copied from WhatsApp. For example, she might receive a message like "(Some insult here), look what I've found: (Some message copied from a WhatsApp chat here)"**
A and B are located in different towns. A is using an iPhone, B had an iPhone at first, but changed to an Android phone later and changed his number at the same time (which didn't help). Both are using recent versions of WhatsApp with end-to-end encryption. B (at least) was also using some web client for WhatsApp.
It's highly improbable that someone had physical access to either phone or to local WiFis.
Is there some common explanation on how the stalker might have gained access to my friend's chats? Is there anything I could recommend to them other that switching to a different app?