For his seniority, he is right to seek proper employment law advice about his exit.
If he has no clients to take with him, a good option is to flip sideways and go in house, particularly if he specialises in a certain sector. I would suggest he start searching for roles inhouse asap as that can take a while. If he joins a big organisation, there is a lot of scope for advancement and remuneration. The best way if he is a partner is to parachute in as a GC or close to GC. It will be a steep learning curve but if he enjoys management and does not like client entertaining, he will feel much more at ease and motivated. There is nothing more soul destroying that not having work to do and still have hourly targets. The best thing about inhouse is there are no time sheets!
If he prefers to stay in a law firm, he can also work inhouse as a professional support lawyer or in Risk or the latest buzz Legal Tech. There are senior non-fee-earning COLP and MLRO roles that sometimes go to ex-partners but they are quite rare.
Alternatively, if he wants to stay in practice, he can consider joining a virtual law firm like Gunnercooke or Keystone. There, he eats what he kills and keeps the majority of his billings. But he must have at least some clients of his own to make that work. There is an internal referral network within these firms but he will have to go out and make those contacts (call it internal schmoozing). There are no targets or external pressure on him or management crap, other than finding and doing the work. Some people prefer this freedom to working in a law firm and is equivalent to self-employment.
The downside about stepping down to join another law firm as a fee-earner as Counsel or as Senior Associate is that unless he takes a big pay cut, he will be very expensive and sooner or later, the conversation will come up re: making partner. If he understands the concept of "Finders, Minders and Grinders", the latter 2 don't come up that often to lawyers joining externally unless your dh has a particular niche skill that is difficult to find.
It will be very useful to know which area of law your dh practices in. There are lots of lawyer mntters who can offer advice.