Yes, it is the most important aspect when proving one's employment. They base those responsibilities to make sure it matches the NOC that your friend has chosen.
Check this thread to know alternatives that applicants have tried http://www.canadavisa.com/canada-immigration-discussion-board/nail-the-challenge-of-getting-reference-letter-from-current-employer-t397796.0.html
It is absolutely mandatory. That said, if he has been unable to obtain it after trying, he can try to submit alternate documents in its stead, but then it depends on the VO to accept the substitute documents instead, and that's not always a given.
It is mandatory to proove your job description tasks. Ideal way is to get it on that reference letter. However some bigger companies do also have internal documentation where all positions have stated job description. In that case you can use bridging referencing to those internal standards (bigger companies under ISO 9001 or ISO TS 16949 or others).
Some companies even have those tasks written in a separate addenum contract with their employees.
Point behind is that you need to have a solid proof of your job description and that reference letter is the easiest one.
