My friend is on probation for underage consumption in Minnesota and just got another one in Wisconsin. What will happen to her

If she's around 17-20 they may charge her as an adult, she may serve a little jail time, she will pay fines and likely be on probation for a long time.

Suggestion: get a good lawyer. I mean, really good. If the lawyer's decent enough, and the situation surrounding the incident isn't too bad (no drugs, no vandalism, no crimes, no one got hurt, etc), then she may get off with doing ~100 community service hours and ~$3000 - 4000 fine.