have over 1 year experience now
You think it's a good idea to reapply with a better job letter ??
The job letter is the most important piece of evidence. If your friend IS in fact working as a supervisor, and can obtain a letter that supports that fact, a new application would be considered on its own merits. SenoritaBella is 100% correct.
Does anyone know what will be the odds of winning this time
It is not a lottery. You either provide CIC with proof that you meet the program requirements, or you don't. Your friend applied as a retail supervisor but didn't provide evidence of performing supervisory duties. It is only a "gamble" if you are not sure that you meet the requirements, or if you are trying to game the system.