This is a very incomplete answer, but important in regards to terminology and understanding everything: RGB and CMYK are color models. They don't define what your monitor or printer can do, only how color is created. A great question to review: What is the difference or relation between a Color Model and a Color Space?

He also told me CMYK is a 100% subset of sRGB.

CMYK is the color model. sRGB is a color space. These are incompatible ideas.

But, for a moment, lets suppose that a CMYK color space is being used, such as a RIP to an inkjet or an offset color printer. Can this CMYK printer profile be contained entirely inside sRGB? That is, is the gamut of the CMYK device smaller than sRGB?

That needs to be determined for each device in use. In general, the CMYK color model (and output devices) are typically thought of as much more restrictive than what an RGB device is capable of. But this isn't entirely true or a complete picture.

Many monitors display the gamut of the sRGB color space adequately. Some display the Adobe RGB space completely. Few delve further, into the realm of what can be contained in ProPhoto RGB. Output devices are equally varied: a 4-color inkjet printer is going to have a relatively small small gamut, where a 7-color inkjet will have a notably larger gamut (cyan, magenta, yellow, and black inks vs cyan, light cyan, magenta, light magenta, yellow, black, and light black inks). Similarly, prints can be made with 8 or even 16 colors. In fact, inks can even be specially mixed -- printing with a variety of blues and purples can also overcome the relatively small number of blues that a traditional CMYK device can output.

So, it is entirely possible that your designer friend's CMYK output device/methods fall within the realm of what the sRGB color space can show, but that's hardly a true or representative statement.