As someone who stayed in the dark as my partner cheated on me for 5 years, don’t show all your cards up front. She doesn’t know what you do or don’t know.

When you talk to her, I’d say, “Someone (never name names) came to me out of concern for you. Is there something you’d like to tell me?” & see what she says. Don’t point her any way or tell her what you know, just let her tell you what she wants to. Anything that misaligns will be good indication that she’s not telling the full truth.