There isn't an addiction mechanic in this game is there? Like I know that Ian can get labeled stoner for smoking weed twice, but we haven't seen any flags beyond that for taking drugs?Realistically, she will do what the player chooses. I imagine between Billy and Ivy there's a decent chance for optional drugs as well, so things can get out of hand even for more restrained Lena. (though, again, if the players opts so)
Either way though I suspect that if drugs are involved with Billy's photoshoot, and Lena has taken MDMA twice, that she probably won't have the option to say no, even if she has a will point. Perhaps if other drugs are involved she can turn those down. I'd be interested to see another scene with Emma offering Lena drugs, just without the easter egg game crash sequence.