Might need to get vaccinated chicks from now on if that's the case. I'd take them up on the necropsy, but not let them to your place. But that's just me. It's probably just fine, but I don't let others hang out with my flock or bring in any birds which have been exposed to adults of another...
If the hen got sick in a few days and died... does not sound like Marek's, which usually affects young birds, is a slow paralysis and then death.
That said... I'm not a fan of big brother myself.