This has become a bit complex because of how things have happened...
What type of tenancy were you on? I'm presuming a rolling tenancy, so she should have given you two months notice, but you agreed to move into her other house with just one months notice? Do you have this agreed in writing somewhere (text, email, etc)?
What was the agreement on the new house? Presumably you didn't sign a new AST?
How long were you in the new house?
The above are important for working out when you were legally liable for the rent. Your liability ends when your friend paid, on the 11th, but to dispute the week before that, you'd need to prove that either you had no liability to pay then, or that you agreed to waive the rent from the 5th - rather than from when your friend moved in. However, has the landlady actually asked you for the week? Because it appears that she's trying to take it from your friends bond, and therefore thinks your friend should pay.
The deposit issue is also confusing. If it's been released from the scheme, it should have been returned to you, rather than the landlord. The scheme should pay it into the bank account you nominated at the start of the tenancy. Was it correctly registered? Did you receive it and pay it to the landlord?
It would appear that your involvement with the deposit ends here, anyway. Your full deposit was returned to you and you decided to lend it to the friend to use as her deposit. The landlady is now saying that your friend owes her a week in rent and therefore the bond is short - this your friends bond, and not yours.
She could not dispute your friends bond during the 30 day period, because that was your bond - and she is disputing your friends bond. It might be the same money, because you've lent it to your friend, but that's a private arrangement.
If your friend agreed to move in on the 5th, and pay X amount in bond, and the landlady has received the bond and rent - 1 week, she is right to let your friend know that this is short. Your friend would need to prove that the tenancy didn't start on the 5th but on the 11th because of her agent.