My friend just told me he is taking a class with a professor that
founded a company with a device similar, but distinctly different from
mine, and I would love to talk to him about this during his office
NDAs have their use and their place, but don't take it personally if this person categorically refuses to sign your NDA.
The more a person is an expert in an area, the more he's likely to have consulting jobs/advisory roles with companies in that same technical area. Also, the more a person is an expert in an area, the more he has heard similar ideas to your idea, or the more he will work with others with similar ideas.
So for an expert, signing an NDA related to his field of expertise usually has not upside whatsoever (unless he's getting paid for it), since through no fault of his own, it may limit the kind of paying projects he'll be able to accept in the future, or it may increase his potential legal exposure.
I speak from experience here. I am not a Professor, but I am a technical expert in my field. And I have no problem signing an NDA if it's about molecular biology for instance (since molecular biology is a field I know little about), but if it's something that is too close to the kind of work I am doing, or too close to the kind of work I may doing in the future, I'll stop the person right there.
Like I said, NDAs have their use and their place. But if you're about to approach a possible competitor of yours to tell him your entire idea. Then please don't. Don't do it for your sake (in case he's not trustworthy), but also don't do it for his sake as well. By not telling him your idea, then you can never accuse him of having betrayed your confidence or having stolen your secret sauce.
Instead, patent your idea, protect your idea, or get it started on your own. Once your idea is patented, or once your idea is publicly out there already, then you can go to him. Or if you want to go to him before that happens, then go to him with cash in hand and pay him for his advice. Paying him for his advice is the only other way he may be willing to sign that NDA for you.