OK, split it down the middle. If you're already a @bob follower when you start following @alice or when @alice un-@s @bob, whichever happens later, then @alice's action has no effect on you. Even if @bob has been hidden from you by @alice's action, it only affects internal-to-twitter links like retweets. If you have a link directly to @bob or his tweet from Google or from an email, you can follow it successfully, and that action removes the hiding effect for you. These extra complications would probably be good for @alice, since it would make it much less obvious to bullies that she had taken this action in the first place. (Yes, if @bob has observant friends, they'll notice eventually, but that's probably OK.)

I agree with you, the main point is that @alice doesn't have to smell @bob's shit anymore.

The point isn't to nail it down exactly right here and now. The point is that this is a discussion that should have taken place many times at Twitter, and we should see some concrete results by now, rather than fluffy PR-focused fake-internal memos to the effect of, "ok guys now we gotta try hard!"