the problem is not the open relationship, so don't listen to the people telling you to move on. you move on and end things with if you want to.
noooow, she's addicted to prescription drugs. that is very worrying because it really is just a step from prescriptions to hardcore stuff. and that is dangerous because what if she does move on from pills to needles and gets an infected needle? no needle is clean or properly sterilized when it comes to addict. i mean it. what if she gets low on money, goes on withdrawl and then robs your bf blind to score money for her drugs?
it would be a tiny bit better if she was functional, but it sounds like she isn't. she can't keep a job, that's not good. what if she gets your bf hooked on these things?
seriously. i dated an addict, it was hell and scary and i spent my days in anxiety and maybe i am overreacting here, i know, and i also know it doesn't look like such a big deal now but trust me, these drugs will stop working, she'll go for harder stuff and i really hope your bf is not being dumb enough to share personal info or lend her money.