Is it bad to use the same interview instruments? It is often recommended to use instruments that have been previously validated (e.g. satisfaction questionnaires). The rationale behind: that these instruments have been thoroughly validated and you should not try to "re-invent the wheel".

However, I've found myself in this scenario: I'm conducting qualitative research with semi-structured interviews very similar to a recently published study. We seek to explore the same topic but with different populations (i.e. hypothetically, they are studying apples and we are studying beets).

Since we are studying the same topic, is it okay (and is it even encouraged) to utilize their interview instruments? Part of me believes strongly in the original statement, "don't re-invent the wheel", but part of me feels like its close to plagiarism and that I should come up with my own questionnaire/instruments.

Our paper, if published, will likely be read by the other authors.

Note: For those who might ponder "why are you conducting such similar research" or "why don't you let them study the beets too", the research we conduct is possible because of access to "privileged" information. In the same way we might not have access to their tomatoes, they wouldn't have access to our beets.
You need to give credit where credit is due. So if you use their instrument, be open about that and cite them. 

Just another reason for reusing existing instruments: You probably want to compare your findings with the findings of the original author. In that case using the same instrument is very beneficial.