My goal for any job interview, no matter which side I’m on, is to end up feeling like I’m talking to a colleague. Colleagues come into my office all the time when they’re stuck on a problem. I ask my colleagues for help when I get stuck myself. So in an interview, I try to recreate that dynamic.
In other words, what would you say if a colleague needed to implement a fibonacci sequence and didn’t know what it was? You would explain it to them until they grasp it enough to continue on their own. There’s no shame in ignorance as long as it’s not permanent.
If you go through that exercise and still can’t picture yourself working with that person, then they’re not a good fit for the job.