So, is this person correct, does using the key for encryption only once make using a predictable IV safe?
Yes, he is correct.
This attack works by the attacker obtaining a CBC-mode ciphertext, and wants to verify the plaintext contents of a specific plaintext block. To do that, he computes that the input to the block cipher would be if that guess was correct (and the corresponding output), and based on that, create a plaintext message that, with the predictable IV, send that input to the block cipher. He then asks for that plaintext message to be encrypted with the same key. If the corresponding ciphertext message has the expected output block, he then knows that his guess was correct.
If we encrypt only a single message, then he cannot perform this attack - he can learn the original ciphertext, but then he cannot ask for a second message to be encrypted with the same key.
Now, this holds if the encryptor takes the entire plaintext, and encrypts it in its entirety, and that full ciphertext is sent to the receiver (and the adversary). If you run CBC mode incrementally, for example, you take the first part of the plaintext, encrypt it, send it out, and then take the second part plaintext, and then encrypt that, then that's not safe - if the attacker can listen to the first part of the ciphertext, and then (based on that) modify what the second part of the plaintext, he can implement the 'predictable IV' attack - even though it may not look like you're sending a second IV, because of how CBC mode works, you effectively are.
Also, in this instance, an attacker could not control the entirety of the plain text that is encrypted, just a small portion of it.
Doesn't matter - it's still safe even if the attacker has arbitrary control. For example, if the adversary can specify the entire message except for one bit, he still can't learn what that one bit is.