AFAIK that is correct. I think the P4 is indeed the form you fill in to close your existing business.
Until he submits the form, his own micro entreprise won’t be considered closed and therefore he won’t be able to register to work with you in yours.
Also, just to make sure you don’t get any more nasty surprises further down the line - I assume you are aware that cotisations will be due for him as a conjoint collaborateur; details here
In order to correct any misunderstandings - I am not suggesting that anyone “break the rules” be they Gendarmes etc or lowly folk like you and I.
Just sometimes, a good discussion with all concerned, can throw light from a different angle - it happens - and suddenly one can see a way through the mist.
Sounds as if Teresa’s situation was/is not such a simple one as perhaps it seemed at first glance.
Let’s hope it all gets sorted satisfactorily asap.