What one might consider being the ethical answer, is not always what the legal answer is.
You didn't ask what should one do ethically, you asked how one should answer the interview question. Even if you personally feel that you would risk violating HIPAA to tell your friend, you should know its illegal, therefore it would be incredibly stupid to admit to this in an interview.
In the situation you gave, you do not have the legal authority to release the information. Your legal options would be to consult the police (not that you would tell the police about the partner being HIV positive, but if your friend is not mentally capable of making decisions and under a guardianship, or is underage, ie a potential statuatory rape issue) or to consult the public health department (I don't think public health departments routinely get involved, but if the partner is promicious and not being honest about his/her HIV status, then possibly the health department would see it as a public health issue and get involved.)
Personally, I don't consider it my responsibility to tell a mentally capable adult what they should be doing. If my friend is an adult and educated enough to know about disease transmission and safe sex, then I do not feel I have any responsibility to tell them their partner has HIV, especially considering it would be illegally in the situation given, for me to do so. If my friend does not know about disease transmission or safe sex, then I would certainly try to educate them....after that, its their decision.
For all I know, my friend has already discussed the situation with his/her partner and is aware of the partner's HIV status, and has decided to have a relationship with this person anyway. People DO make this choice-and probably not as uncommonly as you might think.