Refer to the CIC Instruction Guide and it will give you an idea of all the points that MUST be included in the reference letter from the employer. Look carefully at the job duties of the NOC and be sure that your wife performs them in her line of work. Do not copy them word for word, but apply her duties as they fit. Give a copy of the requirements to the employer so he/she will know all the points (start/end date; salary; hours; job duties, etc). Letterhead and business card.......Good Luck!
http://www.cic.gc.ca/english/pdf/kits/guides/EG7.pdf