You see, integrated GPUs are (obviously) structurally different to normal cards, intuitively, so is their software. Most video game engines are not coded to be efficient with integrated GPUS.
They may be quite different structurally, yes, but the OS hardware abstraction means that it presents the same interface to any software written to use a graphics card. In this case, that would be the graphics APIs and libraries used by the games (DirectX or OpenGL), which further abstract it to a unified interface for the game itself.
What you said is still substantially true though: integrated GPUs generally support fewer features which means that the graphics APIs and / or driver have to emulate them (or just drop the feature altogether) which can affect performance a little or a lot, depending on exactly what is being requested of the card.
And some games just flat out dont work with integrated cards.
Like, seriously.
Quite true. If the card doesn't support a needed feature and the driver can't emulate it, you're SOL. This was common back when vertex and pixel shaders were a new concept, for instance.
AND, dont forget. Because the integrated graphics IS your motherboard [? (sometimes just your CPU, not sure with this model)], if you push it too far too often you may completely root your comp.
While technically possible, unless he's overclocking the GPU I doubt he's got much to worry about.
And in the unlikely event that the card does go south, it
probably won't damage the rest of the motherboard. Just get a replacement and hope you can get into the BIOS to tell the motherboard to use the inserted video card instead of the integrated one.
Anyway, I suspect that it really is just the integrated card that's at fault. Depending on what games you're talking about specifically, some hardware features might not be present but needed in older games that newer games don't need, for example. Cue driver emulation and performance going to crap.
I'll give an example of this: I have Aliens vs. Predator on my desktop right now. I also have a new Radeon graphics card (don't recall the model, but it's got 2GB of RAM to put it in perspective), but AvP runs at a terrible frame rate with horrible graphics glitches. It was written around 2000 and uses DirectX 7 I think. The video card driver just doesn't work well with it. If I revert to a much older driver, the game runs fine. Same story with Starcraft and Diablo II: they're so old and use legacy display modes that modern cards don't handle them well, even though they can run modern games full blast.
So your options are somewhat limited. You could try changing the driver out to an older or newer version and see if it helps (not terribly likely). Or you can get a new graphics card.