Eastling,
You have 3 choices.
1. Keep the cph for all the holding including her bit. This will count as a single holding, which has two implications for you.
a) Your name is against the cph, so you are responsible for maintaining the records and for any regulation breaches, incluidng anything she does wrong, so you pay the fines or go to jail!
b) The standstill periods apply to the cph, so her moving goats on would create a standstill on your pigs, and you moving pigs on creates a standstill on her goats.
Overall not a good idea
2. If she has an existing cph against land she has elsewhere, she could add the bit of land she is renting from you to her existing cph if it is within 5 miles of the cph she currently has. This can only happen in practise if she can move stock on/off this land without needing to unload on your remaining land, otherwise there is no benefit and indeed it is worse, as unloading on your remaining land would create a move to you, and an instant standstill, meaning that you would have to hold them for 6/20 days before moving to her land. It is ok if she has to drive over your land to get to hers, as long as she loads/unloads on her bit.
This option would mean
a) she is responsible for her own records, and you yours
b) her standstills would not affect you, and visa versa
c) She could move animals between her two areas of land without creating standstills on herself
3. Get a further cph for the bit you are renting to her. This would be the best option if she doesn’t have an existing cph. If she already has a cph elsewhere then you have your existing one, she has her existing one for her other land, and a third is created for the bit you are renting to her. As with option 2, this can only happen in practise if she can move stock on/off this land without needing to unload on your remaining land, otherwise there is no benefit and indeed it is worse, as unloading on your remaining land would create a move to you, and an instant standstill, meaning that you would have to hold them for 6/20 days before moving to her land. It is ok if she has to drive over your land to get to hers, as long as she loads/unloads on her bit.
This has the advantage that it would be easy to bring this back into your cph at a later date if she stops renting, or to transfer to another person who might rent it after her.
This would mean
a) She has her records, you have yours
b) Her Standstills do not affect you, and visa versa
c) But she would put a standstill on herself by moving between her existing land and the bit she is renting form you
Option 3 is the best option for YOU, and option 2 is the best option for her.