Wait... "His victim is him victim"? :> I'm afraid you messed something up a bit.
I will assume that you meant "Jego ofiara jest jego żoną". Or "Jego ofiarą jest jego żona". Both would work, actually.
Well, in general both work in almost any sentence. The difference is that using "to" is kinda like saying "X = Y", so in many sentences the more... descriptive "jest" will sound a lot better.
We would discourage using "X to Y" if X is a person. (Though some sentences like that are probably in the course anyway). I can't say that "Mój brat to żołnierz" is wrong, but it really doesn't sound great. And we would reject that construction if X was a personal pronoun. "On to żołnierz" really sounds too clumsy.
Maybe you've seen it, but I will link anyway: https://www.duolingo.com/comment/16373167