DEAR MISS MANNERS: How does one correctly respond to the greeting "Howdy"? Is it to reply "Howdy" back, as in responding to "How do you do?"
GENTLE READER: The origin of the word "Howdy" (with alternate spellings, such as "howedye" or "how d'ee") dates from the 16th century in southern England. As Miss Manners recalls, it was used, as American Southerners do now, to morph the greeting, "How do you do?" into a colloquial contraction.
So yes, "Howdy" requires only a reciprocal "Howdy" -- and perhaps a mirroring of its enthusiasm depending on how deep in the South one finds oneself.