It depends. If the story is positive and your mom is okay with it, then it's probably fine. For example, if it's a story about how she overcame difficulties, it can be inspiring.
This is completely unethical. Having sexual relations with a friend's mother violates many social and moral norms. It can cause great harm to the friendship, the family, and all parties involved. It shows a lack of respect for boundaries and relationships.
Perhaps it means that you are making use of a story related to your mom in a creative way. For example, if you are writing a story together, you might include an incident from your mom's life to add more depth or authenticity to it. Maybe your mom has an inspiring or funny story that you and your friends find useful for whatever you are doing, like creating a skit or a short film.
Since I don't know the details, here's another possibility. It could be a story related to her hobbies. Perhaps she has a passion for gardening, and she transformed a small patch of land into a beautiful and flourishing garden. This not only brought beauty to the neighborhood but also became a place of relaxation and joy for the family. The story could detail how she learned about different plants, how she dealt with pests, and how she shared her love for gardening with others.
Her best friends could be the people she has known for a long time, like her old schoolmates. They share memories from their youth and have a deep bond.
Well, you can expect a lot of complicated emotions. Your friend might be shocked or even angry at first. Socially, it's quite an unusual situation. You'll need to be prepared for some awkward family gatherings. Also, you'll have to handle the age - gap related differences in lifestyle and values very carefully.
One possible event could be that she moved to a new city and had to adapt to a whole new environment. She might have had to make new friends, find a new job, and get used to different cultural aspects of the new place.