Narcissistic triangulation is a where someone with strong narcissistic traits, brings a third person into the relationship to remain in control.
This third person can be an 'outsider' to the existing relationship, of someone known too/close too both in the existing relationship, who hasn't been used in this way before.
I don't think you are being unreasonable OnwardandUpward. You are more sensitive to this type of game playing than someone without your past experience would be.
When you think about it, we often come across a 'friend' who complains about and/or criticises a mutual friend to us which is why it's not uncommon to think we need to exercise a degree of caution, because if 'a' is doing this to us about 'b', they're more than likely doing the same thing with 'b' about us.
As you are unsure that your concerns are valid, just be careful about what you say and discuss with this friend. If there's something you think the other friend may be upset or offended about if you said it and it got back to her, don't say it.
My motto is never say anything about anyone behind their back that you wouldn't be prepared to say face to face.
Just be careful and see how things pan out.