You might even ask him to elaborate on various points.
Sorry, this is a lengthy reply, but even if you dismiss my initial advice, I suggest you continue reading, as I make a series of related points you should consider.
I'd definitely ask him to elaborate. Even with beta readers, who are assigned the task of finding plot holes or problems with a story the author never anticipated, they need to be prodded to explain precisely what bothers them about a particular passage.
Once that central issue is identified, you can take one of two approaches, either flat out fix it, or if you know the story eventually resolves the issues, you can buy the story time by having your character acknowledge the problem (ex: say by having someone sit the main character down, and telling him he's got a problem, and have him (the MC) justify his actions. Once readers know it's an issue in the story, those concerned with that issue will often wait to see how it plays out.
Here's a classic quote for you:
Remember: when people tell you something's wrong or doesn't work for them, they are almost always right. When they tell you exactly what they think is wrong and how to fix it, they are almost always wrong.
- Neil Gaiman
Note: his comment about readers being wrong about what's wrong is why you need to press them, as their first complaints usually are, which is why you continue to press them, because their initial complaint ("This story is going in the wrong direction") often point to another, more specific complaint, they aren't publicly voicing, and it's only when you drill down that you'll discover the underlying condition (though their being wrong about how to fix it rarely fits a particular story).
However, a separate problem is, it NEVER pays to respond to a negative review in public, as it makes you look like an ass to the entire world, and rarely changes anyone's opinion. Everyone is entitled to their own opinion, and if you disagree with someone's direction for your story, the best option is to suggest other stories (hopefully yours), they might prefer instead. That way, you remain true to your story, but still retain a future appreciative fan.
By badmouthing someone, you're more inclined to turn off anyone Googling you, who may be trying to decide whether to read (or buy) your stories. It's NEVER a good idea to engage trolls. Everyone gets bad reviews, and an false accusations or bad advice almost always speaks for itself (i.e. your other readers will recognize it as invalid, so you don't need to make an issue of it. In fact, most reader automatically discount any reviews that are all positive, as they assume they were all written by the author and his minions (i.e. friends and relatives).
Finally, as a follow-up on Gaiman's thought, while everyone's opinion has merit, they don't always represent the majority opinion. However, that said, you never know how many readers you'll eventually lose by refusing to address their concerns with your story. Thus, tread lightly!