I wonder if the letter might be better coming from your parents? In our friend's first attempt to get a TRV, CAIPS noted that there was no financial evidence provided from the invitor (I never dreamed that would be necessary!!) If you are an unemployed student, that might be difficult to produce. The invitor can provide a notarized copy of BIO page of passport/birth certificate, 2-3 months bank statement, income tax return, thus showing they are residents of Canada and financially stable. Have your friend also prepare a written statement of the reason for their visit to Canada. VO must feel there is a "compelling reason for the visit" and that the person "will leave" as evidenced by "strong ties to their country".
Take time to go through as many posts as you can that discuss rejections so you can avoid repeating other's mistakes. All the best.