I worked with someone like this (direct supervisor) who was also a friend. It was hard. It got hard for me too when people realised I was friends with her.
I was pretty young- first proper f/t job. I worked there as a student p/t, became friendly with her then, when I didn’t have much to do with her work-wise. She very definitely went out of her way to make friends with me, which I would have been wary of if older/wiser as it was so concerted.
In the end, I did leverage my friendship a little by standing up to her when she was wrong and standing my ground. Tried to get across to her when she was being unreasonable. Didn’t tell her what other people thought of her though.
Moved onto my next job pretty quickly and stayed in touch with her as friends. She later moved jobs and subsequently offered to get me a job there (very prestigious place). I declined which surprised her.
She was very successful there but pissed a lot of people off. A mutual friend talked to me about it and asked for my opinion (I think at her behest). She had been called into her bosses office and told that whilst she got great results, he’d had a greater number of complaints about her than anyone in his whole working life. In fact, he’d had more complaints about her that year than about everyone else in the department put together (and it was about 100 people).
So I told mutual friend that yes she did make people angry, because she never did any teamwork, she treated a lot of people as inconsequential/beneath her, she didn’t listen, she was perceived as rude. That usually she did get results but people hated her. And that that might be dangerous in the future for her, because at some point she’d need help from colleagues and it wouldn’t be forthcoming or she’d make a mistake and no-one would have her back, or she’d treat someone she thought was unimportant badly and actually that person would have more clout than she realised or someone important would take exception to her attitude.
She apparently did try to cultivate a gentler work persona after that, but it was pretty shallow. She became a bit politer but she didn’t really start helping others or being a team player.
Her one friend at work (someone she’d gotten a job, who was similar in style) made some mistakes and got fired. Then she was pretty nasty to the PA of someone very senior in the organisation (not her boss or his boss, but someone very influential). He had a good relationship with his PA , witnessed part of the nastiness and decided he wasn’t having it. Made it clear she wasn’t to speak to anyone like that again.
And after than she was basically frozen out over about a year and she left. She spoke to me about it and I actually advised her to go broad for a fresh start, as her work was of a very high standard and she’d get a fresh start re work relationships/reputation.
She was very successful there, but again her working relationships deteriorated. She left and formed a consultancy with someone with a much better way with people. It was successful and grew, employing about 20 people. Who then had enough of her and she was forced to sell-up to her partner, or the partner would quit and set-up on her own everyone else would quit and folllow her. So basically, have some money lose the company and the name, or keep the name, have no money and no company.
She’s restarted a consultancy business on her own now, working on much smaller projects. She got a pretty good sum from the partner for her share of the business.
She also has a child now, she and the dad have never lived together and aren’t involved anymore, but he does co-parent. That was basically a convenient arrangement too- both were late 30s, successful, wanted a child, say one another as the right calibre and had a short relationship with the express purpose of having a child.
She was minorly off hand and patronising to me one time a few years ago. It was just the final straw for me though and I read her the riot act.
She got in touch a little while later, said she knew we’d had words but she still had a lot of fondness for me, could we pick it up. But I was done. I hear about how she’s doing occasionally through mutual friends but we pretty much inhabit different worlds now.
Because people don’t really ever change. She gets what she wants and it doesn’t matter to her how she does it. She is happy- she has money, a child and interesting work.
If you think your friend would listen and learn, say something. If not, the only advice I can offer is leave.