This post is an extension of this previous situation that I discussed a few years ago here: Supervisor has said some very disgusting things online, should I pull my name from our paper?
That advisor contacted me (and the other collaborators of that paper) today, saying he's submitting his PhD dissertation this month, and that one of the requirements is getting signatures of all co-authors related to the dissertation. My natural inclination is to not sign anything involving him, however I'm not sure how strict this requirement is, and whether or not my not signing would significantly impact his dissertation.
Personally, I think his views are at odds with his field (psychology), and I would prefer that he doesn't earn such a degree and gain that level of influence. However, I'm obviously hesitant to do anything that would stop anybody's life's work; I'm not in the business of ruining lives. These two preferences are opposing, which is frustrating.
Ideally, signing/not signing his dissertation would have no impact on anything, and then I can happily not sign it and continue to distance myself from him. But I don't know enough about academia to determine if that is feasible.
My question is therefore, do I run any serious risks to the advisor or myself by not signing his dissertation?