This is covered in our help center:
All solutions to challenges should:
- Correctly implement the required specification.
- Be a serious contender for the winning criteria in use. For example, an entry to a code golf contest needs to be golfed, and an entry to a speed contest should make some attempt to be fast.
Our policy on invalid answers dictates that they should ultimately be removed, but not before giving the poster a chance to fix the post.
Considering this answer was posted by a new user, it's important to be welcoming, explaining the scope of the site and the purpose of code golf instead rather than citing rules and threatening deletion. Suggesting how the code could be made shorter and maybe a link to the corresponding tips question could be helpful too. I wouldn't cast a downvote at this point, but that's a personal choice.
That being said, the most likely outcome of this scenario is that the user never comes back, since he never intended participating in a code golf contest. If that happens (the poster didn't respond to the comment or you find an ungolfed answer that is over a month old), please flag it for moderator attention.