Do Helpers also need to set up two-factor authentication?

Yes. One of the reasons that all Block Party users are required to set up two-factor authentication is precisely because there are second-order effects when someone's account is hacked. Someone who has added Helpers to their account would also be at risk if one of their Helpers had their account compromised. This has been an attack vector on other social sites; for example, someone gets into your friend's account and then can view your data. On Block Party, we require that everyone set up two-factor authentication for that extra layer of protection.