As a follow up, looks like I am out of luck if I want to not take the last segment. Customer service said my companion wouldn't be allowed to fly without me on the last segment for "Security Reasons".

They did offer to rebook me for $400 in change fees to make it work.

A bit disappointed, as Alaska has always been so flexible when I've had problems in the past. One would think not taking a last leg with no need to change to another flight would be so difficult for them to allow.