I'm sure that will be comforting when he's out £999, or however much he's planning to lend her.Could try trusting her, and if she doesn't cough up, then, well, you know who your friends are when it comes to money.
Neither a borrower nor a lender be,
For loan oft loses both itself and friend
Lending money to friends and family can become very complicated if what is essentially a business agreement becomes tangled up with a personal relationship.
If you're happy to never see the money again then trust them with it. If it's too much for you to lose then some kind of formal contract/agreement is necessary.

