Do you think I should tell my girlfriend to stop smoking weed?
Why would you tell her to stop smoking weed?
Since she smokes weed, she cannot achieve anything else in her life
It's fine to care about your gf's health. It's reasonable to ask her to stop doing things that you think is bad.
Yeah I totally care. I want to stop her from wasting her life away.
Giving her the encouragement to quit would be a good thing if you believe she is hurting herself. As her boyfriend it is completely valid for you to worry about her life.