You will not get a perfect cleaned-up version of the audio you want. There are some techniques to try, which may improve the sound.
Firstly, gate the audio- Whenever the interviewee is not talking, mute his audio. Only unmute it whenever he speaks. (This may need to be done manually, and you can decide if it's worth the effort.) Alternatively, you could duck the interviewee's audio by the interviewer's, but you will lose the interviewee whenever the two people talk over each other. This can be done fully automatically in any DAW or Audacity (with "auto-duck" effect).
Secondly, in principle, since you have a track which is the noise you want to remove (the interviewer's audio), you can use this information to filter out the noise better than if you only had the noisy track. I can recommend one free plugin called "kn0ck0ut", which runs as a normal VST plugin in any DAW. It takes two signals (1: noisy signal and 2: noise) and subtracts the spectrum of 2 from 1.