Army book > rulebook.
However, as Red... states, 'accompany' is an undefined term.
She cannot buy a COB as an upgrade, as there is no price mentioned for it. So she is not replacing the DH.
Let's suppose that 'accompany' means 'join'.
- It must be agreed by the opponent, since the rulebook never uses 'accompany'.
It wound not be wise to count on that interpretation during a tournament, better ask before.
- Crone must be on foot, of course
- Category:
What would it be?
To count the whole combined unit as a warmachine, that would be too harsh: only the COB should autofail S or I test, but Crone should be allowed to pass the tests.
On the other hand, counting the whole unit as anything other than warmachine would be too cheesy: why would an accompanied COB become suddenly immune to autofail tests and impassable terrain?
Therefore, the category could only be "unique" (p.87), requiring detailed explanations as below.
Basically, this could be applying all warmachine rules to the warmachine part of the unit, and all infantry rules to the Crone part of the unit.
- Formation:
is it a skirmisher formation? This would explain the word "accompany" rather than "join".
However, the skirmishing rules would apply only for the formation, being charged, and the free reform.
The combined unit could not claim any other effect which would make the accompanied COB better than the lone COB, so no benefit when being shot at (too cheesy), no benefit for any terrain (too cheesy).
The alternative is that Crone and the COB would be set in contact base to base, and the COB attendants would be placed anywhere (they are irrelevant for any effect whatsoever). There would be no rule problem. However, that would not explain anymore "accompany" rather than "join".
- Movement.
No way it would move like skirmisher, because that would be too cheesy.
To make the whole unit move like a warmachine (impassable terrain, no march...) is the only reasonable option.
- Ward save:
the COB has it, not Crone. Rationale: the DE army book says that the COB has a ward save, not the unit.
- Magic resistance:
Crone has a special rule to cancel the first spell cast at the whole unit with 50% prob.
- Shooting:
as p.99, half of the shots go to Crone, half to the COB, the odd one is your choice.
- Template shooting:
as p.99, hit both if both are under the template; no Look Out, Ma'am.
- Charging from the unit:
as p.101, Crone can leave the unit and charge, naturally.
- Charging the unit:
this is blurry, the unit is not anymore only a warmachine. Let's assume it is like charging a regular skirmisher unit (narrowing the formation).
- Challenges:
Crone is able to challenge and answer challenges.
- In close combat:
The opponent chooses if he hits Crone or if he hits the COB. A max of 6 models can hit the COB. No maximum exist for Crone.
- CR:
of course, the unit is stubborn. It has the Ld10 general and the BSB.
As you see, there are some blurry fields left over by the word 'accompany' interpreted as 'join'.
This is why the wisest would be not to pay attention to that word, except for friendly fluffy home games.
EDITED to include the unit category and provide more details.