The reporting verbs would normally be in the same tense, that is, either asked and responded or asks and responds, and there is no point in using the adverb colloquially if you are reporting a conversation. The verb here would normally be reply rather than respond, it would normally follow the speech, and separate speeches will often occur on separate lines. I don’t know what NM means, but if you think your readers will know, then you can use it.
Some may disagree, but a comma after a reporting verb isn’t really necessary. There is also no need to add a full stop (or period) after a question mark and terminal quotation marks. It is clear where the sentence ends without it, and it only adds clutter to the page.
Taking those points into account, I would suggest:
He asked “How do you do?”
“NM” she replied.