It is possible that her bf may have access to her whatsapp by mac spoofing.He may know the mac address of her phone.
MAC spoofing is a technique for changing a factory-
assigned Media Access Control (MAC) address of a
network interface on a networked device. The MAC
address is hard-coded on a network interface
controller (NIC) and cannot be changed. However,
there are tools which can make an operating system
believe that the NIC has the MAC address of a user's
choosing. The process of masking a MAC address is
known as MAC spoofing. Essentially, MAC spoofing
entails changing a computer's identity, for any
reason, and it is relatively easy.
The changing of the assigned MAC address may allow
the bypassing of access control lists on servers or
routers , either hiding a computer on a network or
allowing it to impersonate another network device.
MAC spoofing is done for legitimate and illicit purposes
He might have spoofed his phones mac address and assign hers.This way whatever replies she gets will be received by him......but still the question is that if he knows the mac address