Whether or not he asked for adjustments is irrelevant. If they knew he had a disability, they should have at least offered adjustments to his role, responsibilities or working environment.
You need to dig out fit notes, emails etc that reference his anerusym as if you did seek legal advice, they are very likely to argue either that he isn't disabled or that they didn't know.
You have several options:
1 - leave, which should be your last resort for financial reasons and because it weakens his position in the event of any claim. constructive dismissal is an option but very hard to win.
2 - see a lawyer, perhaps on a no win no fee basis, for disability discrimination and constructive dismissal if you go down that route.
3 - take sick leave until he finds something else, bearing in mind this could lead to them dismissing him for absence but if disability related (and depression/anxiety are disabilities), this could strengthen the case for discrimination
4 - stay put and job hunt.
5 - submit a formal grievance which could be a long drawn out and stressful process.
I am doing 2,3,4 in my own case (except my lawyer is through a union). I have successfully negotiated the continuation of full pay during my absence by referring to meikle v nottingham council.
Its very stressful but the one thing my lawyer was absolutely clear on is DO NOT QUIT. So take sick leave and job hunt.