I've been debating the validity of reductionism with a friend for a while [...] Does anyone know an explanation of reasons to reject consciousness-causes-collapse that isn't explicitly predicated on reductionism?

This quite possibly can't be done. If you handicap yourself by refusing to use an idea while examining its merits, you may well draw inferior conclusions about it, and modify it in a way that makes it worse. You should use your whole mind to reflect on itself (unless you conclude some of its parts are not to be trusted). See these posts in particular:

• The Lens That Sees Its Flaws
• Occam's Razor
• A Priori
• Where Recursive Justification Hits Bottom
• Which Parts Are "Me"?