As with all things, it depends.
In general, it is useful to have a network of colleagues and former colleagues that you can go to when you're encountering problems. There are a few colleagues that I'll hit up periodically (and that hit me up) when one of us is struggling with a problem that we suspect the other person might have some insight on. These are generally something very similar to a good StackOverflow question where the person doing the asking does all the legwork to ensure that the question is clear, concise, and answerable. If that's what's happening here, it's probably a good idea to reply if you can because building your professional network generally pays off over time.
On the other hand, if your former manager is telling someone you don't know to contact you and the request is more along the lines of "can you spend some unknowable period of time helping me debug exactly what's going on here" rather than a well-researched, specific question that is respectful of your time, it's entirely appropriate to decline to sign up to be free technical support. Commonly you can come back either feigning a lack of memory (i.e. "Sorry, it's been 8 months since I've thought about that system, I'm really not sure where to go to start debugging that") or, if you're amenable to it, approach your former manager to see about doing some consulting (i.e. "It sounds like the Foo is Barred in System X but it's hard to be sure from the description I have. I'd need to Baz the Frobbit to be sure. And un-Barring the Foo is likely to take Y hours. I'd be more than happy to help you work through the problem in in the evening and the weekend but I'd need to charge you $Y/hr with a Z hour minimum").