1 2 3 4
On consulting an attorney, in many American cities the local ... rights groups - they're more likely to have the knowledge.

If there was a "no pets" clause when he bought the condo, and he signed the contract containing the "no ... Others in the building signed that contract with the assurance that there would be no pets in the building.

It's not a clause. It's a by-law. When a person makes an offer to purchase a condo they are given a copy of the by-laws. Mine was literally an inch thick.
If, however, it's a new ordinance, he may have a case.

The Board of Directors doesn't just arbitrarily pass new by-laws. Something like this would be put up to a vote of all the co-owners. If you had an "animal" they would be grandfathered in. There is no he "might have a case."
As for the comment that a cat isn't even considered a pet..??

X-No-Archive: yes
The Board of Directors doesn't just arbitrarily pass new by-laws. Something like this would be put up to a vote of all the co-owners. If you had an "animal" they would be grandfathered in. There is no he "might have a case."

I didn't see the OP mention anything about the pet being grandfathered in. It sounded as though it was a notice about not having pets, period. Would be nice to know if that "by-law" was in the original contract, and if so, why send out notices at this point? I've never lived in a condo so don't completely understand how they do things.
Becky