How to comfort your girlfriend when she cries ?

My girlfriend just got divorced she feels really bad she was only married for 6 months she was with the guy for 2 years. She was cheating on me with him she feels stupid. She has a son from her first marriage his father died 2 years ago he doesn't know about the divorce yet he's only 10 she's not sure how to tell him. She feels really bad for cheating she doesn't have feelings for the guy anymore. How can I comfort her through this ?