Speaking as a BS, I'd say turn down the invite and be okay with the reality that some relationships (even great relationships) just fade into the past.
I wouldn't be comfortable knowing my WH was meeting with someone who could give/share "updates" to and about his AP.
Maintain "Hi/Bye", be cordial, and invest your time with new friends who aren't a threat to your marriage.
I realize this friend hasn't done anything wrong, but she's a "door" nonetheless. My WH has had to close many "innocent doors" in order to give me the security I need.