I’ll give the same advice I would to friends who want to marry outside the Church, especially non-Christians of whatever religion:
Emphasize that the Church will make her, the Catholic party, promise that she will raise as far as possible her children Catholic (and she should be planning for the possibility of children - it’s how the world works). She has to promise to remove any danger of her becoming non-Catholic.
And then, on his end there are problems. As a Muslim his faith, too, requires promises from him. Islam obligates the father to take charge of religious education in the family. That of course would be contrary to the promises she would have to make to be married in the Church. So one of them is inevitably going to give in, and the Catholic cannot do that. So it would render a spouse betraying their faith. Not a recipe for a good marriage.
She says he’s a Cafeteria Muslim. While it irks me that people treat their faith like trifles, let’s assume that. What if he becomes more devout? It happens that people of any religion might become more devout as they start having children, sending them to school and so forth. You have a fundamental religious disagreement. Now I’m not saying he’s going to beat her up or anything or honor killings or anything like that (which is definitely a minority in Canada). But that’s asking for continuous fighting over religion. She will have to attend mass with her children but without her husband. And children will ask, “Why doesn’t Dad go to Mass? Why don’t you go to mosque with him on Fridays?” There is a great risk for indifferentism, which should horrify any person of faith, Christian or Muslim!
You say he is a bad Muslim, however. Let us suppose he really does treat his faith as if it were nothing. Does she want her children to see faith as something so trifling that it’s not worth living according to its rules? That’s even worse than if he was practicing! If he treats God, who is greater than anyone and anything else, like that, that’s a red flag.
In short, the Church might grant a dispensation. But she must make the promises, or no priest in Montreal will even consider marrying them. And she should think really hard about the implications of marrying a non-Catholic, especially someone who holds his own religion and God in such low esteem.
I’ll be sure to keep everyone posted.