Tash,
I have a hard no on a number of people but that is because I have large houses in an urban location and tend to attract parties. I have various ways of monitoring this and guests are informed.
She is not going to stay in a caravan, she will go inside and we all know it, they will find a place inside. I could be wrong and it does not really matter at the end of the day.
She is being honest, most would have sneaked the person in so honesty is appreciated.
I would say in your case you should just do it.
As for insurance, you have Aircover. They are not great as far as damage, but have been great with personal injury/liability (based on personal experience and account from others)
Do you have independent insurance for your property? If you do not, it is a must.
You can have her sign a very simple waiver, something you can find on line. it might not hold full legal weight but will give make her more responsible.
In your answer, i recommend mentioning that your property does not have infrastructure for a caravan as far as adequate lighting etc so this should be at her own risk
@Chris-and-Natasha0