What makes you so sure that it will "live much longer" that some disease wont come into the tank tomorrow and kill it because its immune system is shot due to being in the environment it lives in naturally,
you are arguing one case against many that have been stated on this site, stop arguing your single fact against many arguments you are making Canadians seem ignorant
Give us the specific breed of your teachers eel and we can look up life expectancy, dont say "many" then we can look up the life expectancy of that breed and see how long into its life it is.
Next off just because one has lasted that long doens't mean all them can, who knows maybe this one singular eel for some reason is resilient to fresh water, why are you arguing the FACT that this is a brackish eel and keeping all brackish animals in freshwater is unhealthy