I recently bought my first THC pen after reading
@Teevster's article about always having your place stocked with stuff useful for getting/having girls over (which also includes bunch of different alcohol, snacks, condoms, lube etc). While I am like you in that I don't want to indulge in marijuana regularly or even semi-regularly, I regret not getting it sooner because it is an easy "in" with stoner chicks.
If this girl is giving you an easy way to get her alone with you (smoking weed), I'd say go for it so long as being around weed is acceptable to you. Don't worry about precedent; this girl seems like she already is a stoner, so either bang her once/twice or make her just a fuck buddy if stoners aren't your ideal girlfriend.
You also don't need to actually smoke with her. You can instead drink, or once she is over tell her you decided to stay sober for whatever reason, or even fake taking a hit.