💯
Bullying doesn't just affect the target- although they are undoubtedly the worst effected.
Where bullying is widespread & is permitted, the workplace culture is rotten to the core and everyone is being let down by leadership.
Bullies don't stop being bullies if their target leaves, they will redirect their nastiness at someone else when it suits them.
If you're friend is as lovely as you say and from what you've said about these people, then I'm guessing much of it comes down to her face not fitting and, because she's soft,they know they can get away with it.
You can support your friend and back off, while continuing to raise the issue. Any employee who witnesses bullying and other inappropriate behaviour has a moral duty to report it to HR.
Bullying in the workplace destroys lives. I've seen talented, strong-charactered people absolutely broken by it over just a few months. There are numerous stories on this site about posters or their partners who've been victimised this way and it's taken them years to recover.
Your friend might have a level of inner resilience lacking in others, but it will get her too in the end. I'd keep encouraging her to leave AND demonstrate how unhealthy it is by seeking a new job elsewhere myself.
This is a fight she's never going to win, it doesn't matter how she changes her approach or work-style, they've made up their minds and it will always come back to bitchiness and abuse. No one can really develop when they're being constantly undermined and shut down at every turn. She'll blossom elsewhere.
If you are really prohibited from reporting the behaviour each time, then start keeping a log. Don't save this on the company computer or systems, and make sure you have a backup. Document everything - and if you leave first, provide a copy of it in your exit interview.
She'll need it one day.