No, it is not - it is unhealthy only when practiced in large close-knit groups because it leads to genetic disorders from inbreeding. That harm usually builds up / accumulates over centuries. But it is not set in stone and there is incest in the bible etc. - the taboo is not biological in nature.

My late mom was a passionate dog person, but she only had an affinity to mongrels while finding purebred dogs despicable due to their stupidity. It turns out that she was right - genetic diversity is a good thing (mongrels manifest genetic diversity).

But in this case, the bf is an adult man who is successful in major areas of living and who is having non-procreative sex with his biological mother who did not raise him, is not an authority figure vis-a-vis him, and is using sex out of her desperate fear of abandonment. Plus, he uses OP's pictures while his mother is pleasuring him, and, wants OP present (perhaps mother knows about that desire of his). So in essence there is a web - a family structure, however unusual.

The therapeutic approach called "family systems" seems to be the answer here, and within that approach, practitioners probably are well aware of GSA it is the first time I hear of GSA and I assume that most lay people have not heard about it).

A "family systems" therapist who could see all three of you together would probably be best. Maybe if the mother gets some reassurance that sans sex her biological son and his gf would still keep her in their lives would release her from the fear of abandonment that fuels her instigating the son's desire.

I think that a family systems approach is admittedly daring and demanding, but it would be far, far quicker to yield results than individual therapy for him. If he is in individual therapy, his mother would be hell bent on undermining its results. If the therapy is joint and the mother gets to have positive reinforcement for not instigating him - say, OP+bf+his mom go see a movie together - then perhaps she at least would not be undermining but neutral with respect to therapy.

I agree with OP that the issue seems to be extremely narrow and that there is little reason to suspect that this man would cross other societal boundaries, especially given that he is over 40, has had several LT r/s, is well loved by the adoptive family, etc.