When a man tells a friend that she is a kryptonite to them what does that mean? Is it good or bad?

-kryptonite was superman`s biggest weakness... so in that sense the girl is his weakness...he just cant live without her...his biggest weakness.kryptonite was superman`s biggest weakness... so in that sense the girl is his weakness...he just cant live without her...his biggest weakness.Read this, and you'll understand:

http://en.wikipedia.org/wiki/Kryptonite
It means she's killing him.