What's the proper thing to do in this situation? Should I flag as "not constructive / off-topic" or just let it go?
According to the post which states that "what have you tried" is no longer proper:
… comments that consist of little more than "what have you tried" can be deleted with a single flag.
What have I tried
I answer a question every once in a while on StackOverflow. I would say that I am well aware of the comments including "what have you tried" or with the same intention, and the situations involved in them.
I usually ask for an edit by stating what I feel others would need to know to be able to properly answer the question, a helpful hint, a generally accepted standard or guideline, or what I feel is lacking.
In my opinion, at the heart of the "what have you tried" issue is what is known as a "help vampire". It seems to me, "what have you tried" usually stems from a reluctance of users to blatantly do the work of others. This is where it is important to provide guidance on how the OP can share more details or more requirements to narrow down what is usually a rather vague question.
Unfortunately, sometimes the OP really is trying to just get some very specific work done for them here without trying themselves, and in that case I feel a vote to close as "Too Localized" is appropriate if it is unlikely to help anyone else out (for common situations I believe it can be understandable to show a simple implementation that others can edit and re-use).
As stated in the linked material, if the question is beyond saving it can always been downvoted and closed. Ideally, it would be improved so that someone can answer it.