Some repliers insisted that it should be whatever Bob wants, which is trivial. It sidesteps the issue and doesn't address the question at its own level.
I asked, what should be the outcome? Not, who should decide?
What options should Bob have to choose from? What should be the default? What should he choose?
Anyway, I agree with the vast majority that the reply should be to Alice's followers. I think the rule of thumb for replies is that they should address about the same audience as the original post, or optionally a subset of that audience. Expanding the audience confuses readers and violates privacy expectations.
If you have a fediverse account, you can quote this note from your own instance. Search https://cosocial.ca/users/evan/statuses/116036025096704223 on your instance and quote it. (Note that quoting is not supported in Mastodon.)
