You have some good responses but let me add my $0.02.
He's convinced that it's carbs that got him overweight.
That very well may be the case for him. Dietary intake and its effects on people is a very subjective issue. I have specific problems with food no one around me can understand and that's fine, I don't understand their way either. We all have different backgrounds and experiences, even when it comes to food :)
So I try to convince him that it's all about calories in/ calories out
It would be all about CICO if it wasn't for the fact that specific nutrients can actually change our BMR without our realizing it. If you know your exact BMR everyday, it is indeed CICO. The typical CICO equation does not (in fact, cannot) take this adjustment into account, which is the basic explanation of some people might claim to beat the CICO equation. It is in fact possible that under specific circumstances BMR can change drastically. Our bodies are not a machine, they're magic! :)
Based on genetics and lifestyle factors, carbs do not affect everyone the same. Some people can develop some form of food addiction, and due to "faulty" hormonal workings, not respond well to eating a high carb diet on a regular basis. Some people do not experience these and they thrive on carbs. It is very subject-dependent, so both you and your friend are right. A single way of eating will never be the right way for everyone.
Let's have a little fun with CICO:
1- I personally know ballerinas who eat ~800 calories a day. They practice for hours EVERY DAY. They're not lying or underestimating, as they barely eat to begin with. By typical CICO, they would have reduced to 20lb by now :)
2- CICO is not be all and end all. If you overate a measly 20 calories every day, you would gain 10lb in 5 years. Can you imagine the ridiculousness of overeating "20 calories"?? :) There is no way anyone can control such a thing or count calories that precisely. You don't actually gain weight unless you begin significantly overeating because your BMR adapts to resist change.
CICO estimates can be a useful tool to lose weight, but they're just estimates, as long as we keep that in mind, all is fine.