Perhaps I'm extrapolating too much but if the guy is asking you specifically, "Do you think it's a good idea?" then either he's very early in his idea, he hasn'

Perhaps I'm extrapolating too much but if the guy is asking you specifically, "Do you think it's a good idea?" then either he's very early in his idea, he hasn't thought through why HE thinks it's a good idea or he doesn't have the confidence that his idea is good. If it's the first two options, then he needs to put more time into developing the idea to gain the confidence that it's a good idea. If it's the final one, then perhaps it's not worth pursuing the idea or he's not in the right position (confidence) to do so. Of course, it's always smart to solicit advice on an idea such as the potential challenges, risks, strategies etc.