"A junior who makes a fuss is going nowhere fast. Unfair but true, especially in law"
This is so true OP.
It's been a long time since i qualified but the competition for seats was still cut throat, even though (back then) there were almost enough jobs for every trainee in the cohort.
I'm afraid that one sniff of you being 'difficult' and you'll be out on your ear.
I'm going to disagree with the previous posters who said to speak to recruiters and try and move on qualification. I wouldn't. It only ever looks like you didn't get offered a role at all or a role in a competitive department / team (unless you go from a banking firm into an IP boutique and it's obviously a passion you followed).
Try and stay at your firm 1 or 2 years. Once you are qualified and usefully competent / capable you're a hot commodity, you should have no difficulty getting another role unless you're in a niche team.
If it makes you feel any better, in a weird way, almost every large commercial firm is like this. Drama, one upmanship, backstabbing, espionage, treason are the hallmarks of every partnership and it almost always flows into and infects the culture. The business model is literally cultivated from possessiveness (of clients), ambition (to outperform your fellow partners), fear (of losing revenue, and thus status and relevance - literally, as your equity errodes) and greed (to earn more to compensate yourself for the terrible work life balance).
If you move firms you would be quite unlucky to immediate encounter a similar character but they're there and they will find you, or your friend, or your boss/partner. Sorry to say that is corporate law!
So no, do not jump on account of this one twat partner who is clearly feels exposed and knows he has a target on his back for not bringing in any work. He or she will be there in the next team, or the next firm, i promise.
Keep your head down, your opinions to yourself and support your friend but do not get involved. I've read your post and note that i don't think you were actually present or involved when any of these things happened to your friend (apart from the incident where the partner called after the ex-trainee, which in context was evidently a barbed comment but on its face is completely innocous) so honestly, there is nothing you can or should do here apart from take your friend for a drink regularly and let them have a moan.
(Despite the above, i love my job but only because i found an ally - a partner - to whom i'm 2iC and we look out for each other and our team and our clients, but no one else).