Just a quick question for anyone that knows. Next week I will be house-sitting for someone who I told about Steemit and he has an active account. If I post from his I.P, will it get my (and his) account locked?

The F.A.Q states:

Am I allowed to create more than one account?

Each user is allowed only one paid-for account created via Steemit.com, however users are allowed to create multiple accounts on the blockchain. Creating additional accounts on the blockchain requires users to pay their own account creation fee for any additional accounts.