Help, mystery solver needed! Question re using original number on new device with different number

HI all,

I know there are various ways you can do this however my question is a little more complex...

My friend received a load of inappropriate pics from a man she was buying a house from
She replied to him telling him to leave her alone
He replied a few hours later from that same number saying: 'Phone has been stolen, pls call me on my other number: XXXX'
My friend replied to question him
He respomded from that same original number
Hours later, his old account is not showing as a contact on whatsapp, but his new number is
My friend said that the whatsapp convo has remained the same, but that his number had changed

So my questions are:
Is it possible to use your old account on a new phone that has a different number, without having the original phone to verify this on?
And if your number changes, surely you would need to start a new whatsapp chat message? How could it auto update?

Is this guy trying to play dumb?

Help!
C