If she's a daily smoker, she doesn't need to be around her friends to partake.

It's a *habit,* not a physical addition like with an alcoholic or an addict. It's easy to quit if you choose to, doesn't matter how many years you've been smoking. I know what I'm taking about here, my drug experiences are varied. I'm a long time smoker, one who quit 3x for extended periods, for specific reasons (2 pregnancies, a job), no problem. The first few days are hard, I was a little cranky, but once past that, didn't even think about it.

The question, really, in gauging whether or not she'll quit is how come she smokes? Boredom, stress relief, a type of self-medicating?

"She told me that losing me wasn't worth continuing the marijuana use."
My advice - Give her the benefit of the doubt, tell her you want to believe that is true, not being a smoker you don't understand the appeal or what it will be like to stop and you're concerned that instead of really quitting she'll hide it from you. See what happens. Trust she's telling the truth unless there is clear evidence to the contrary.