Lots of people see patterns of misdeeds in other users. Only some of them are actually right. In no way am I asserting that you are one of the wrong ones. But because there are plenty of wrong ones, and because your meta question raises some red flags, I'm going to play the devil's advocate, lest this meta question ultimately become the rallying cry for a small number of users to flag every post, comment, or chat message they ever disagree with.
First of all, it's not always necessary to know the answer to post constructive comments. It's possible to help troubleshoot a problem without knowing the answer. And it's possible to help clarify what someone is asking without knowing the answer; sometimes, this is possible without even knowing much about the topic being asked about. Asking "a set of follow-up questions" hardly sounds like an evil deed!
Nor is knowing the answer required before downvoting posts. Especially if all these incidents occurred in questions that shared a tag, it may simply be that an active user in the tag responded to multiple questions. If your friend's question was closed, that took action by five close voters (3000 reputation or higher), or by a moderator. Unless this user is a moderator, they did not get your friend's question closed all by themself.
Now, it's totally unacceptable to ridicule or attack people over their off-topic, not constructive, duplicate, or whatever-other-close-reason-applies questions. But it is acceptable to post comments that argue that a question should be closed, or explain constructively why it was closed. The standard for whether or not a comment is appropriate is not whether or not the author of the post being commented on is happy it was posted.
I don't know the real nature of what has happened, of course. I know that a lot of people get angry when people downvote their questions or comment to say that their questions should be closed. I am the target of that anger, sometimes. I have even often been attacked for downvoting posts that I never downvoted, because I commented, and someone else downvoted. For all I know, the person you're talking about could be me!
You say the commentator was "obnoxious" but all I really know is that this person has upset two people, who are friends with each other.
It's not that I distrust you. It's just that every active user who tries to be helpful is, from time to time, cast as the sort of villain you're complaining about. So, maybe you've thought through whether or not what they are doing is really inappropriate. Or maybe what they're doing is so clearly inappropriate that you're chuckling as you read this, knowing if that if I knew the details then I'd agree wholeheartedly and wish I didn't know them. Or maybe it's not so simple.
Whether all of what I've just written is something you need to read, I do not know. What I do know is that it is something many people need to read. (Not necessarily these precise words, but the general idea I am trying to express with them.)
How to flag a pattern of bad behavior by a user:
To report a pattern of bad behavior on the part of a user, flag one of their posts for moderator attention, and write a custom message. Your flag will be kept confidential (it will not be shown in the 10k tools, only to actual moderators who have agreed to respect your privacy). If the problem lies in comments, you can flag a question or answer that the comments have been posted on. This should also be a custom flag, and you should make clear that the problem is not with the flagged post itself, but with some of the comments on it.
For individual bad comments, you can flag them. But even if you custom-flag them, there is no guarantee that a moderator will read the flag, because with several flags from the community comments disappear. Custom-flagging a comment is good when it's wouldn't be clear from any of the other flag reasons why that one comment should be removed. Flags on posts should be used if you need to communicate something about a pattern that is occurring, or a user who is acting badly.
Before you flag to tell moderators about someone, please reflect to make sure that the problem lies (at least in significant part) with them, and not with others, and not with yourself. If you're a good custodian (which not everyone is), then most of the time, you can take the time to ask yourself this, and quickly you will see that in fact you are right and you should flag.
Finally, please note that sometimes, no one has acted in bad faith, but people still have managed to create a problematic situation. When this happens in comments, often the best solution is for a moderator to delete the whole big wall of comments. Comments are "second-class citizens" in the Stack Exchange system, and if they're not-constructive, rude, off-topic, chatty, or obsolete, they should be and (whenever people notice) are removed. Sometimes a conversation between good people who are all trying to help each other out starts out good and productive, but manages to end up being all those flaggable things.
So sometimes when no single side is specifically at fault, it's still worthwhile to flag, not to call upon moderators to investigate and deal with any specific user or users, but simply to clean up a comment conversation gone wrong so that a question can become productive again (or for closed questions unlikely to be reopened, so that it can at least rest in peace).