Seeking information about the commercial kitchen hood cleaning industry

I'd love to learn more about this fragmented, required-by-health-code industry from anyone who has direct knowledge or has even looked closely at a hood cleaner for sale.

I own a service plumbing company that does a lot of drain and sewer cleaning. Last week at an event a jokester introduced me to another business owner with the line, "The two of you have the grossest jobs of anyone I know... you should compare businesses." He is a retirement-age owner of a 30+ year-old commercial kitchen hood cleaning business with zero knowledge of how to sell and exit a business. We have many mutual friends and he's a solid guy, so I've agreed to spend some time with him to cover the basics of SMB transactions before he enters the sale process inredactedIt's not a large business... 2 vans, 4 employees, and ~$700K in revenue. SDE ~$200K. He handles sales and administration and rarely goes out with his crews to clean.

To best help him I'd like some background knowledge on this space. I'm also starting to ponder the industry as another vertical for my own portfolio and wondering if there are examples of successful aggregations of a handful of these companies in a single geography to make a more robust business.