This is an extremely awkward and inappropriate situation. It goes against the norms of a healthy family relationship. You need to communicate your discomfort clearly. You can say 'Mom, this is not a topic that is suitable for discussion between us. Our conversations should be positive and appropriate. I hope this never comes up again.' If possible, try to distance yourself from such inappropriate conversations in the future and focus on building a more appropriate and healthy relationship.
We should avoid such inappropriate topics. Instead, we could focus on positive family stories like 'times when mom showed great kindness' or 'instances of mom's selfless love'.
No, it is completely unethical. Having any sexual encounter with a friend's parent violates multiple boundaries of respect, friendship, and family values. It can cause irreparable damage to the friendship and create a very uncomfortable and immoral situation for all parties involved.
This is a very inappropriate and uncomfortable situation. You should firmly and clearly tell your mom that this is an unacceptable topic. For example, you could say 'Mom, this is not an appropriate thing to ask me. I'm very uncomfortable with this conversation.'