I think everyone has made some good points that I agree with. But I would ALWAYS take it from the perspective of the individual asking. Is he interested in feedback? Is he wanting your opinion to help him get better? Or is he proud of his accomplishment and wanting kudos? Based on that I would reply appropriately.
As many pointed out, it could always be a matter of style. I, personally, LOVE BIG Belgian beers. But most people I know still think Bud Light is the be-all, end-all for beer. No way am I going to worry about what they think of my beer because I could brew a world-class beer (in my dreams) and they still wouldn't be capable of appreciating it. I, personally, don't care for stouts. I can appreciate a good one, but more than one is too much for me. Doesn't mean it's a bad beer, just isn't the beer for me.
Taking all of that into account, I would then use the same "coaching" philosophy I used in th Army and when coaching kids. If he just wants kudos, then I'd find something positive about it and leave it at that. If they want feedback and I'm not a fan of the style, I'd point out that I may not be the best judge. If they want feedback and I usually care for the style, I'd point out the what the bad parts are first, and then offer a few ideas that might address the problem. Then finish with a positive on what did go well and what they did right. ALWAYS keep the criticism on actions involved and keep the positive on the Individual.
An example, "Hmm, that's an interesting aftertaste. You might have let the fermentation run a little high leading to that banana-like flavor. All in all, not a bad first effort. You've obviously got the right color and malts in here. A little more tweaking and you'll put Anheuser out of business"