Assuming something more involved than a quick answer (such as helping
troubleshoot, or domain knowledge transfer that was overlooked etc),
would it seem unprofessional to be willing to help but on a consulting
I guess I'll have to disagree a bit with some of the other good answers. While professionals do deserve to be paid for their work, professionals also do a lot of unpaid work in many circumstances. It probably depends on how much of your time is needed for this type of help, and what type of work you do.
I've helped co-workers on numerous occasions over the years (still do), and would never think of charging my friends or family. I consider this part of my personal networking, part of my responsibility to my profession, and think of it as a "what goes around comes around" scenario. If I ever needed a reference or some help, I know I could count on them.
On the other hand, if they needed a week of my time, I needed to cancel a vacation to help them, or if I needed to postpone a paid consulting gig for their benefit, I would think about asking for a consulting fee.
In my case, I always considered it as "helping my friends". If instead, you feel this is more like "helping their business", you might view things differently. If your job is consulting for this sort of business, it probably makes sense to offer your consulting services more formally.