Gay Men's Challenges Support Group
This community is dedicated to the challenges that gay men face, both between the partners in a relationship and from other influences (family, society, etc.). Find support and talk to others who may be facing the same challenges, and share your experience.
Remember, your ex's mother is not your ex. She doesn't have (or lived) any of what you lived with her son.
A simple thank you should be enough. If she tries to continue conversation, then you may need to think about doing something else. Otherwise, it may just be simple friendliness.