Complex area but in essence, this is UK law (laws for other countries may be different):
Commissioned works
When you ask or commission another person or organisation to create a copyright work for you, the first legal owner of copyright is the person or organisation that created the work and not you the commissioner, unless you otherwise agree it in writing.
However, in some circumstances, for example when copyright is not dealt with in the contract to commission the work, courts may be willing to find that there is an implied licence allowing the commissioner to use the work for the purpose for which it was commissioned. This does not necessarily result in a transfer of ownership. Instead, the commissioner of the work may only get a limited non-exclusive licence. This situation demonstrates the importance of establishing who owns copyright through a contract.
In short, you need to get her to agree to sign a model release form if it's for public consumption. And if she knows its for a porn site, then I suspect she'll refuse.
Why would you want to film a sess with a hokker and post it online? Is it for bragging rights?