[Vautrin]

As I see it, the sole reason for remaining in the relationship is his child. A child that is used as a pawn by J, to push her agenda. Add to that a lot of fears, instilled by her, and perhaps some ignorance of Canadian Law (I assume it is at least a bit sane on matters like this).

Try to find out, about the workings of Canadian law. What would happen, if J went to the other side of Canada with the child? Is that allowed? If you can dispel those fears, as they will probably be central to his reasoning. Which justify him putting up with this kind of behavior in the first place.

J's behavior seems excessive for a sane person. What is actually required by Canadian law to allow for (forced) investigations of mental health?

Talk to him, and focus mostly on the child. And the unhealthy situation the child is in, and that it only will get worse, if S accepts the situation as it is.
I don't know S, but if he comes from a background where similar fights have occured, then he is risking raising his daughter in the same environment again.

I can understand if you feel a bit double-hearted about the ultimatum. But his living condition is draining him, draining the business as well. I would not make the ultimatum yet, but persist in unraveling the reason(s) why he claims he puts up with the behavior in the first place.
And remind S of the future of his child.

Find out if spousal abuse / abuse by a partner has to be reported by the abused partner (in some countries that is not needed for an investigation). That might be a last resort, so you don't have to know that yet.