No, only the actual owner of an account, or someone with power of attorney authorizing an action, can change a beneficiary while the account owner is alive.
In addition, your hypothetical makes a great law school question, but that's all. John's statements of his intent regarding not wanting Tara to get the insurance would be hearsay and not admissible to prove that he wanted to change the account. That said, its not relevant. You cannot have to provision changed now unless you can show that Tara somehow prevented him from changing the designation (which would be very difficult to establish).
Sorry, I know that's not what you wanted to hear.
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I agree with Mr. Schultz. This reads like a law school question. I rarely answer questions from "friends." This question is definitely not an exception.
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