This should never come up, at least not while you're both interviewees. Firstly, there's probably some kind of legal issue disclosing employment status of someone else (IANAL). Secondly, it's highly unprofessional; what if, for example, the other person wasn't actually unemployed, then you (the unrelated person) could contact their company and be like "hey, did you know Joe is interviewing behind your back?" That could cause a lot of problems for everyone involved.
It might come up once you're an employee at the company, in which case you should tell the truth. Don't speculate about things like "Joe was fired because..." unless you were actually in the meeting when he was terminated; you don't know what happened and are just making speculative guesses (you can't even be sure he was terminated unless you saw his termination notice, which you probably have not). However, mentioning things you did experience, positive or negative, are fair game.
If this question does come up during the interview:
1) You should not give a positive review. Giving a positive review gives them an excuse to hire him over you. You are both competing for the same job, and if you want the job, don't give him an edge. However, you don't want to give them the impression that you can't work together; perhaps they've already settled on hiring him, but want to know if you have some kind of history. If you don't particularly mind working with him, then you should give a neutral review to show that you can work together without giving him an edge.
2) You should try not to give a review at all, if you can avoid it. Don't badmouth others behind their backs, that's unprofessional. If they press you into answering, you should say something like "I'm really uncomfortable answering this question, but if you absolutely insist, then..."
3) Deprioritize any company that is unprofessional in this way. It's not their business for interviewees to evaluate each other.