UPDATE:
Some of you guys aren't reading the whole thread ( I don't blame you), so here's an update.
After some good advice here I'm not going to be present when she try's it. I've figured out I'm more worried to lose her to a world of drugs than for her to try it. I have no irrational fear of drugs, I have friends who take them, and that doesn't bother me at all. I'm going to stop pushing her away and instead help her explore what she wants to explore. I've told her this now, but I've also told her that if it goes to far I can't carry on. I'm not going to ditch her to ruin her life, but I'll help her if and when I can, but not as her boyfriend.
She seems OK about this, the idea of her trying it at home and not a huge party was hers, and she's happy for this to happen still. She's not going to jump into anything now, and as a result of our 'rut' she'll try and put it off until we're back on our usual rails.