Hi Emilie,
If they marry in the United States, before coming to Canada, they can actually apply for what is called "inland sponsorship" at the border / port of entry. They will have the forms there. If they have been living common law for at least one year and can prove it, then this is also an option if they are not yet married.
If they don't want to marry in the US, but would prefer to marry in Canada (and they haven't been common law for at least a year), then it's a little trickier. She must have some legitimate reason to enter the country, even though she doesn't need a visa. If they say they are going to get married in Canada, and they can show they are engaged, for example, everything might go fine. (Again, they would the apply for inland sponsorship right there and then. But if they are just coming here with some vague intention of marrying some time in the distant future, she should have another reason for entering - working, visiting, what have you. (If she says she is just visiting but they admit they are in a long-term relationship, they may have a little trouble.)
All forms can be found at cic.gc.ca
Hope this helps.