What can I do if I suspect an owner is abusing his cats?

I would caution you to tread carefully. If I were in your shoes I would seek to understand by asking some more questions in a friendly tone first. (such as how long have they had the cat ? how did the cat come to join their household.?)

I am aware of many animal lovers who trap feral cats (in addition to all kinds of other critters that I would refer to as wild animals Not pets) and then rehabilitate them in their own homes, sometimes with the intention of then getting them rehomed through a shelter or a rescue group but that doesn’t always happen. Prior to them taking them in, these wild animals acted as menaces to the neighborhood.

Also, I know people who have taken in dogs and cats who were severely abused, within inches of losing their lives. Those animals traumas which can affect behavior, sometimes remain for years even if the current owner gives only love.