I wonder if the hypercritical young lady actually has Asberger's syndrome, a personality diagnosis on the autistic spectrum. Those I am very familiar with can only see things from their point of view, with absolutely no social filter on their speech. Generally very intelligent as in book smart, scoring well on Mensa superintelligent tests, but socially inept. Much of their "expertise" comes from books and computer searches, but unable to critically evaluate junk from truth. They cannot understand why their criticisms aren't well received because they are never wrong and are merely pointing out how smart they are.
I am glad your friend has someone who understands how difficult her cousin is, and you are able to help her thorough her embarrassment. Your tact was amazing! I'm afraid my fuse is quite a bit shorter.