Where did your friend get the key? Did you also get the installation media from your friend? Enterprise is obtained only through volume licensing.
Does your friend work for a Microsoft partner, or for a company who uses volume licensing? If so, your version of Windows, while being activated, is still not legitimate.
I work for a Microsoft partner, and we use Enterprise on our computers at work. However, we do not use them on our own personal computers. We could easily enough; we have the install files and product key available, but it would be violating the ToS.