I really think this is a non-issue. How often do you give two alternative answers to the same question? I know that it is permitted, but IMHO, best practice is to give one answer. If you think alternative approaches deserves to be mentioned, describe them in the same answer.
But in the rare cases when this happens, it is easy enough for the user to fix. AFAIK, a user can switch to a different accepted answer at any time. I don't think there is more to do than you've already done: Use comment to enlighten the user. It even looks like you managed to get through in the end :-).
As for the (now hypothetical) case that the users mark of approval stays with the "wrong" answer, I suggest you forget about it and move on. The person asking the question is the one that places the check-mark. That is the way it should be. Moderators should not (and do not) interfere with voting or with what is accepted.
There is a lot of (IMHO) wrong answers on Drupal.SE that are accepted and upvoted. One more or less does not make a difference. Here is a rather curious example, since a lot of famous Drupalistas are involved: How can I obscure what I am using to run my site? (I know that author of the accepted answer - @greggles, author of the Drupal security bible "Cracking Drupal" - is just being ironic. But is irony permitted on Drupal.SE? And do all the people who've starred this get the joke?).