New phone and number but the old (still registered) number wrote in a group

A friend got a new phone and a new phone number but the old phone number wrote something in a group chat.

Initially I thought that the number got assigned to a different person and someone could read old WhatsApp message that way, but she said that her contract should still be active until September so that shouldn't be the case.

The message is also a bit strange because the person writing it seems to have some personal knowledge, but it could also be a case of just reading a bit of chat log and making a lucky guess.

My question is if the provider could have messed up and given away the number too early and can someone access the WhatsApp history and chats that way?

Could the old Account have been hacked? Is a WhatsApp Account easily hackable, because the people in that group aren't too tech savvy and while it might be a case of a random person that is a bit less likely I'd say.

Sorry if that has been asked before (probably has) but I searched on Reddit and didn't find anything helpful.