This. Help the person you're interviewing. You'll both get more out of it, you'll both enjoy it. If you're in a company that is expanding, you'll be interviewing a lot, so it's really important that it doesn't depress the shit out of you.
Doing a collaborative interview requires careful attention by the interviewer, but I agree it’s worth it.
I've definitely had interviews with solid candidates who just faced an early roadblock, but were brilliant after I gave them an early push. Sometimes that's more of a sign of communicating a question poorly or nerves than genuine lack of knowledge on the candidates part.
I've been doing a bit of technical interviewing and I usually try to help people along so they reach the "end" of the interview, especially if they're not doing too well. So many candidates sound defeated when they're struggling with a task and it's a form of respect for me. They've put themselves out there, after all.
Treating candidates - which you know you are going to reject at some point of the interview - with dignity is the right thing to do and can go a long way when it comes to your company's reputation.
Usually it is not hard to notice whether you'd like someone to succeed and then factor that in.
What about the opposite, that you don't like someone, or worse, you are indifferent to their success?
Maybe the hard part is to get a HR person that cares about the outcome instead of their authority.
Why should their interview be a stressful stone-walling event when life as an employee is not?
Maybe an interview should have "Who wants to be a millionaire" lifelines. Two "Googgle"s, and an "ok, I give up - give me a hint".