Whatsapp Security Issue

I have a question regarding the relationship between devices and individuals and the rights of access to WhatsApp messages

I used to have a work phone, on which WhatsApp was downloaded and used for both personal and business purposes. When I returned the phone to my former employer, I wiped and re-set the phone to factory settings.

However, when another colleague was passed the phone for her usage and re-downloaded WhatsApp, messages for a group that she was not invited to (a personal group of mine), started to receive messages. The admin for this group neglected to remove the old work number, so she kept receiving the messages, which she was clearly not meant to be party to.

She then proceeded to take information from the group (an email address and password) and log in to my private G-Suite drive, accessing my private data and documentation.
I am in the process of taking legal action against them for identity fraud, so could you please tell me what other laws and privacy regulations they have breached , for accessing my personal information from the platform, when they were clearly not invited to the group as an individual?

Anyone? Bueller...?