Troubled by a 'friend's' move on my close friend - Advice needed

I have a friend for over 9 years. She is 38, married with a 13 yr old boy and I am single and 32. Ever since we have known each other we have been friends. I never had romantic feelings or intimate thoughts about her(I guess I am not that much into married women). Over the course of years since 2015, we have had a very close friendship and at a professional capacity I am tutor to her child.(The child has been hanging out with me since he was 4). Me and the lady went for vacations and have spent countless nights on the balcony with a drink and lots to talk about our lives. I am thoroughly aware of her troubled marriage including instances of DV and her complicated upper middle class family dynamics. She knows my childhood, how I lost my parents and has been close watcher of how I have transformed over the years. In 2020 in a moment of my weakness, loneliness, desperation I spoke to her extensively even breaking down and she somehow made the call the treatment me like a son and I have ever since addressed her as 'Maate'. (My mother passed away in childbirth so my knowledge of a mother's presence is next to 0) During the pandemic where we could barely meet during to distance and lockdown. Her husband also moved to UK for work. A new "friend" comes to the picture. I did not meet him at the beginning but after a few months, I notice my friend taking care of the finances, lifestyle choices of the"friend". He enjoys the involvement citing how difficult his life was where his parents could not provide such interactions when he was a kid. (The "friend" is 28 years old). The "friend" also a leukaemia survivor indulges in alcohol with us, tries other substances in her company and one night confesses his feelings to Maate. Maate tells him that she has a kid, a husband and a boyfriend so those spectrums there is no space for the "friend". So the "friend" officially friendzones himself but over the times has arranged him to stay in her place, sleep in her bed, cuddle with her everynight(can't sleep otherwise) has access to her emails, photos, phone password, and subtly starts taking control over her house to get things done his way. He even does not allow the 13 yr old child sleep with his mom because the child gets a pole in his sleep(like of teens and men) it creeps the "friend" out. Finally after a night of drinking I suddenly woke up to sounds of moaning early in the morning from her bathroom. So the "friend" finally had his long overdue sex at 6 am in the morning in her bathroom. I wake to listen to Maate moaning buty paranoia kicks in when I see her kid waking up and standing behind me and asking 'where is Mamma'. I have no words, I have no idea what to do. I take him away on the pretext of making some yummy breakfast. Now the problem for me is: 1. I have lived by a few codes and one of them is not to cross boundaries with female friends. I have stayed friends with them for over 2 decades. So someone doing it infront of me and calling it friendship and apologizing with the words 'heat of the moment','honest mistake', 'drunken daze', etc just makes me call it bullshit. The "friend" wanted it and took the first shot he got. 2. My Maate asking me to let it go, forgive and treat the "friend" like a younger brother. I have tried it a lot over the last year and I sincerely can't(because of reasons mentioned in Point 1) 3. Saying it to openly to Maate has starined my equation with her. I just want to stay away from such a "friend" but evidently voicing it out (albeit in a very loud manner) pushed away my closest confidant. The only thing I know is if things get better I can't pull of this pretentious stuff and it will make me burst again. I don't know what to do here.