Most of the world knows "tenants in common" as something usually involving a couple of family members, or maybe friends sharing a vacation home.  But in California it sounds like a different creature, from this 2021 blog post:

https://compasscaliforniablog.com/what-is-a-tenancy-in-common/

I think you have to be there to know what they are talking about.  Something like a poor man's condo.  But you can have 10 people owning a building, and then an agreement among them about who gets to use which apartment? But is your client renting a home, or a business space? Or an Airbnb unit?  If the client is in business and renting from an individual who has the right to collect rent for the property he uses, then just use the SSN or EIN on the W-9 the owner gave you.  Unless, of course, they are refusing to provide a number because of being a TIC, which seems to me to be irrelevant.