Since the number is with you now and the person forgot to delete whatsapp....
you get to see all his secrets.....
but whatsapp messages sync with gmail, not whatsapp number, so you get to see his contacts, but not his messages.
Lesson of the day, if you want to axe a number, remember to delete your whatsapp account first.

to use google drive, you need gmail, thus gmail = google drive la....
but either way, unless the person downloaded whatsapp with the same google account, it shouldn't sync messages...
numbers and contacts though, might stilll be sync'ed