It's not easy to ask someone about their grades. But as a mentor, it's important to ask, especially in a way that shows your mentee that you care, that it's normal to struggle in college, and that you are there for them.
If you struggled in college and bounced back from it, it may help to share your own story. And here are a few more ways to get the conversation started:
Sometimes, people really struggle in college because it's so different from high school. Have you seen this video?
How did you do this semester? (Ask about each class if you need to probe for specific information.)
Were your grades what you expected them to be? What do you think helped you do well? Is there anything you want to do better next time?
How are you feeling about (next semester/finals)?
Have you been to office hours/tutoring/the writing center?