There has already been a question regarding questions answered in comments. There have been a few situations when the question was closed unanswered by the community and then answered by another user in the comments.
What is the general policy with regards to things like this?
Answering a closed question, or giving hints where to find an answer, in comments is almost always okay, no matter what the question is about.
There are a couple exceptions, say totally extreme cases like when the question was about something illegal, or deeply immoral, or asked in bad faith (trolling etc.). But other than that, answering a closed question in a comment is totally okay. I do it all the time for off-topic or Not A Real questions that are good, but simply don't happen to match Stack Overflow's guidelines.
Comments are not where answers go. It doesn't matter if the question is open, even. That's just not what they're for; putting an 'answer' in a comment circumvents the community's ability to curate and moderate that content (comments cannot be edited or downvoted or deleted by the community the way an answer should be).
If the community wanted this question to get answers as answers, they would leave it open for doing so!
The question is closed for a reason. That reason is, in some form or another: the site community does not want the question to be posted there. The community thus inherently does not want it answered there either.
You may disagree with that reason, either in general or in particular. There is a process for resolving that conflict, which you can and should take:
edit the question to address its shortcomings, if any are identifiable
flag the question for reopening, so others are aware of the corrections
vote to reopen the question, if you have the privilege to do so
make a post on the site meta querying the closure and/or requesting support for its reopening
By putting an 'answer' in a comment, you are discouraging the original poster from making any effort to improve their post and encouraging them to simply post whatever they like, knowing they'll get what they want without any care or consideration for the site.
Meh. Leave the comments be. Closing gets rid of posts that will not be useful to others (or has other issues), as that is the primary goal. Closing is not meant to stop the OP from being helped; that is more of a side effect.
So, if a user gets helped in the comments of a closed post, let it happen. It's nothing to worry about :)
If you want, you can flag as "off topic" though.
You might have to ask yourself, from a philosophical standpoint, what is a question? and what is an answer?.
Probably the closed post is not the kind of question, understanding this as the "original post" / "initial post" of a "discussion thread", that the site/community wants but independently of that you might want to help the OP by gifting them an answer independently of the limitations that comments might have. If that is the case, go ahead.
From Pekka's answer to General policies on answering closed questions in the comments?
Answering a closed question, or giving hints where to find an answer, in comments is almost always okay, no matter what the question is about.
On the other hand, if you are looking to help the community fulfill its purpose, you should first check out the corresponding site tour and the corresponding per-site/child meta.
Please, do your best to understand the community's purpose and the community's workings and act according to them.
Many people come to Stack Exchange thinking that the button label "Ask Question" is just a generic label as it happens on many other places on The Web. Many assume that the on-screen help/guidance is just a collection of words that could be ignored without any consequence. Some assume that it's not needed to spend time understanding the site, they assume that the "the site" should understand them.
Remember that Stack Exchange sites are not discussion forums, they don't hold discussion threads, they are "question and answer" sites but not for questions and answers in a broad sense, they are sites intended to be used for a specific way to ask questions and for a specific way to answer those questions.
It's on you how you spend your time on the site / Stack Exchange network, you might take a "hit and run" approach, or you might look at each site as a community garden where the community collaborates to keep them clean and healthy.
