This Judge should have recused himself, once he realized he knew the woman's parents socially.
Also, that was incorrect of the Judge to interview the woman in his chambers.
The ADA doesn't enter into it - this was a very poor, very inappropriate thing for the Judge to do.
That was ex parte communication - unethical and against judicial canons in all states.
Your cousin's atty is in an odd place - if s/he files a Motion to Recuse the Judge, he will probably be sent to a second Judge, sympathetic to Judge #1. But, if atty doesn't get out of that court, the atty is stuck with a biased Judge.
Personally, I'd file a Motion to Recuse & document what the Judge did.
My understanding of the ADA is that it was intended to allow access & provide accommodation to that end... it was never meant to diminish the Cousin's rights or rewrite the law and claim a 'disabled' person was involved, so it is OK for the Judge to violate ethical standards.
One problem I see is that the appearance by the woman is not 'on record'. She shows up, goes into the Judge's chamber & things change, with no record/transcript. Even more reason for cousin's atty to document this unethical act by the Judge - that's the only way it will be memorialized.