Crossing the Canada/US border with a criminal record

I've been kind of loosely planning a wedding for March 2019. (My partner and I aren't officially engaged yet, but it's happening sometime in the coming months, we're long distance so there are people I want him to meet first, but that's neither here nor there). My best friend who's like a sister to me has a criminal record, all for minor stuff, mostly drug charges. She's in Canada (Ontario), as am I right now, but the wedding's in the US (Tennessee), as I'm relocating there and will be living there at that point. I mentioned the wedding to her, and she was like "if I can cross the border, I'm there!" And it made me sad, because like, shit, I didn't think of that. What are the chances of her being allowed to cross the border with a criminal record?