I am not in sales anymore, but did it for almost 20 years at two different companies with two completely different sets of products and services.
My first role was closer to your husbands. It wasn't SaaS, but we provided market data that was consumed by software for modeling. So somewhat similar I think. When I left it wasn't just for one reason. I was a top performer until the end of my time, usually in the top ten percent of sales reps. However I could see the industry changing and I did not care for it. New competitors came into our space, and while we still had the better solution, each competitor drug out the decision making process on my sales efforts. On top of that, from a solutions standpoint we weren't evolving to what people were asking for. So for me, I was spending a lot more time on the sales life cycle than I wanted, to achieve constantly increasing quotas, and eventually got burned out. I can't imagine what SaaS is like right now…it seems like it is exploding with new companies entering the market and at the same time, I hear of a lot of consolidation. So, I left that job for a industrial products role (less technical, more relationship, persistence, and commodity driven) and did quite well for several years until I took a role in another department.
If I can offer advice to your husband it would simply be that positive sales characteristics scale well, at least in my experience. That doesn't mean that any sales role is a fit for every applicant, but if he is hungry, motivated, self reliant, has an eye for solutions and not a quick sale, understands internal stakeholder influence on deals, and can develop relationships, most of the time that person will be successful in a new role, given the company's expectations for new hire performance is reasonable.
Is he open to sales in other industries?
Also, I see a lot of long terms sales reps get into sales operations or even product implementation (if they're really knowledgeable about the software they sell). It is still customer facing but you're not the face of your company and can still problem solve with a purpose without worrying about sales targets.