Why not allow the owner of a Google account to delegate a trusted third party who can handle MFA/otherwise approve logins on their behalf. I kind of do this already by setting the recovery emails for family members (especially aged parents) Google accounts to those that I control, but to my knowledge it is not possible to do the same for the mobile number used to secure the account.
This way, at least as I imagine the authors scenario, the library's regulars could delegate them as the trusted third party, problem solved.
Oh yes, and also what they write -- add an on/off setting for "Be less anal about logins from unrecognized devices".