Even though I'm biased as I worked for a (what was then called a) Big Eight firm, if she can get an internship offer, she should take it.
Assume she does an internship and finds she does not like it and decides accounting is her passion. The downside is that she spends an extra semester finishing her accounting work, but the upside is that the internship will look great on her resume. Again, a lot of 'what ifs' here, but assuming she ultimately goes down the accounting path and gets great grades along the way, she should not have a problem obtaining a position with a Big 4. While she'll probably have to spend time getting her feet wet in the audit (assurance services) area for presumably the first two years, the Big 4 firms and the next 6 larger firms generally all have M&A practices that she could apply for.
I've never worked in investment banking, but I'm assuming the first couple years are similar to those in a Big 4 -- mainly grunt work at the outset, long hours, lots of travel, and dealing with frustrations that it seem like a thankless job. But looking back, I have no regrets with those long and difficult hours in public accounting while I was raising a young family.
One last suggestion ... my oldest son works for a national firm and travels a lot during busy season. But he didn't start studying for the CPA exam until he started working for his firm. Working long hours and then coming home to study for the exam is not easy, especially if married. If she decides on accounting, have her try to plan her schedule where her last semester has just one class - the CPA review course. I'm assuming every university still has these courses. Then she can sit for the exam afterwards and hopefully pass all four parts. Then, she can focus on work without having the added stress of studying and passing a difficult exam.