Our resident Quaesitor has managed to while re-establishing his Parma, fall afoul of a ritual (Summoning the Dagger, from the Festival of the Damned) that has seen him impregnate a woman... whom it turns out is a local witch.
He has scoured the Code, looking for guidance in what he has to do with this knowledge of non-Hermetic Gifted folk, and whether has he to turn the mother of his child in to the Order, or otherwise note the existence of this witch coven, who seem more or less innocuous unless crossed.
Is the "Join or Die!" proscription a part of Hermetic Law, or is it just a tradition/assumption?
It is just a tradition. Technically, you are not required to offer hedge wizards to join - you might just kill them ("Die!"). Or you might simply offer them membership, without repercussions if they refuse ("Join!)". And of course, you may simply ignore them. But the general consensus within the Order is that:
the more great wizards in the Order, the better for all its members - after all, this is the legacy of Bonisagus (note however that a minor practitioner of magic might not "deserve" the honor of joining, nor have enough to contribute) and
that a wizard who violates the code is a danger to all (whether he has sworn to it or not) and refusal to swear is a strong indication that he intends to violate it.
It is a legal mandate. What is spotty is enforcement. In the early days of the Order it was rather stringent. Now, unless something presents itself as a threat or otherwise too powerful not to notice, you can get away with not enforcing it rather easily. But someone else can come along and handle it however they want. They only way to protect your new friends is to get them to swear the oath. Some more stringent types would already have burned them to the ground if they found out they had cast magic on an Order magus, regardless of how the individual magus felt about it.
I don't believe it's a general legal mandate, though it may be in the Peripheral Code of some Tribunals. Even so, I'd be very surprised if it were regularly enforced. The Order did gather all powerful magic users into its ranks, but generally disdains the lesser ones. As I understand it, the reason for Join or Die was to allow the to present a united front, as well as remove rivals and threats. As the existence of the Order as powerful wizards is relatively common knowledge in Mythic Europe, the only real concern would be if a wizard was sufficiently powerful to make his crimes reflect badly upon the Order. Magic (and Faerie and Divine and Infernal) users are sufficiently common that it would be impractical as well as deeply unpopular and definitely interfering with mundanes for a magus to impose the Join or Die question upon every local wise woman, soothsayer and chap who has a knack at finding the best fishing grounds for cod.