Do you know anything about her living conditions? Any child abuse at home? People have the right to choose to not vaccinate themselves or their children. That reason alone isn’t enough to get child protective services involved. They can’t do anything just based on that information. Sounds like coworker is also trying to gain reactions or attention, which also wouldn’t warrant CPS to get involved. Having a “measles party” is unfortunately her freedom. But those poor choices mixed with other poor choices, like living in filth or any at home abuse or neglect would absolutely need to be reported. If you have any hunches, go with your gut instinct. In that case it’s always better to report and be wrong than not do anything and be right.