If I told my coworker who was excited about her son's missionary trip that I thought missionary work is abhorrent and shouldn't be done in the modern world, I might end up getting hauled into HR.

You have a good point there. Also, this is the kind of advice about context about which I think Prudence should have gone into more detail.

Current Friend is to the left of me (in ways I admire) let alone Former Friend. Current Friend has also had a lot of stress (everything from multiple family members dying recently to being a woman of reproductive age in these United States) and I'm not sure how productive heaping this on her would be of me to do. If she hasn't discussed politics with Former Friend that may well be a deliberate decision on her part that I shouldn't challenge.