I'm surprised that so little attention has been paid to Quaestor Lucem's suggestion in comments on the question:
Recuse yourself. Ask your management to find somebody else to interview this candidate. You have a conflict of interest: Given your feelings, it's likely that you will be too hard on him, which is unfair to the candidate. If you hire the guy, some people will think you may have gone easy on him. If you reject him, some people will think you were too hard on him. I would prefer not to have those questions asked about my character, and the interpersonal drama is no good for anybody.
Human interactions are not mathematics. It is impossible to guarantee objectivity when you have an interest in the outcome. Of course, you always have an interest in hiring the best candidate for the job, but that interest doesn't conflict with your responsibilities as an interviewer. This one does.
At this point, if you feel this strongly about it, you can't really be fair. "Try really hard to be objective" doesn't work, because you can't be objective, and everybody knows it. When people play the "how dare you suggest I'm subject to human weakness?!" card, they are choosing to be unethical. That's just make-believe. When you have a conflict of interest, you step away from the job if at all possible, because 1) you cannot perform the task ethically, and 2) for the sake of the integrity of the process, people must be seen to behave ethically. It must be the norm for people to behave ethically. And I mean genuinely ethically, not make-believe "trust me, I'll try to objective, wink wink" "ethically".
Of course, if you live in a culture where favoritism is the norm, you're dealing with a different definition of "ethics". But from what you say, it seems to me that this solution is a good fit for the dictates of your personal conscience.
SouravGhosh brings up a good point: You can create a lot of other trouble by accusing the colleague of applying pressure, and in this case it's not absolutely clear that the colleague intends to apply pressure. If you're going to remove yourself from the interview process, you have to find a non-destructive way to do that. I don't have a very good answer to that part of the question, but I might tell my manager that I don't want to be in the position of potentially having to disappoint my colleague's hopes of her friend getting hired. To me, that doesn't suggest that the colleague is applying any pressure. I wonder if others would see it the same way.