That will only happen if the mixer ends up proving that he sent the coins. With such a threat in mind, Alice would rather indeed sign whenever she has received the output coins (why wouldn't she anyways?), at which point the contract is never been made public as the mixer has no reason to do so and the association between Alice's old and new addresses is never included into the blockchain.  In other words, the "condition that stated x Bitcoins had to be sent to Alice" clause is never released in the blockchain if Alice signs.

However, as gmaxwell points out, it appears that this attaching of a contract and a proof to a transaction is not possible in Bitcoin at this point.