I'm not professional but i think if you told her/him for EX : Ohh i like your artwork but if you draw more you are going to be better .
Or you can say : You aren't good at singing because it's not your hobby , don't be sad ! everyone have a hobby and he should find it , you can try something else and be patient till you find it and i'm pretty sure you will be good at something one day , you should be strong !
Lol i gave you some examples i don' t know if it's will work or not i'm just
trying to help you :) GOOD LUCK ^^