I would first talk to her and mention that you have noticed she has been missing a lot of meetings and work and that EVERYONE has been having to pick up the slack for her.
Perhaps you will even mention that, with the economy the way it currently is, it would be a good idea for her to try to pull herself up and get back to work, or she may be in danger of losing her job.
Also, you could say that you have heard a lot of rumors about her prescription drug use, and you are concerned that she may be addicted. Saying it this way may keep you from being the fall guy when she finds out that everyone knows about it..otherwise, she may know that it all came from you from that trip you took together.
Tell her that some people were considering telling your boss that she is addicted to drugs. Ask her if she has tried to seek help for her addiction. If she has, ask her if she would like some help with her recovery.
If she has not sought help, ask her to please consider having a three-way phone call right now with you, her and your boss so that she may ask him/her if the company has any sort of benefits that would give her time off for treatment. A lot of big companies have insurance to cover addiction treatment programs.
She might be mad at you. I would be surprised if she doesn't get mad...but tell her that you don't think less of her because of her addiction, it honestly could happen to anyone...but tell her you are there for her, and will support her as long as she seeks help.
If she refuses to talk to the boss, make sure she knows that you will. You owe it to yourself to have a less stressful work environment (even if you work from home, she can affect your stress level), and for everyone to do their own fair share of the work. It isn't fair for her to expect everyone to cover for her all of the time, and she needs to hear that.
I wish the best for both of you.