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 kind of topic is not suitable for sharing in a general context. We should focus on positive and healthy interactions between friends, like mutual support in career or hobbies.
This idea is extremely inappropriate because it involves incestuous and vulgar content. Incest is a major taboo in most cultures and moral systems. Fanfic should focus on positive, creative and ethical storylines, rather than creating such disturbing and immoral scenarios.