I've been taking care of my mothers dog while she is on vacation.

I have two cats, the dog will know longer eat her dog food she perfers to eat the cat food. Is that a problem?

No,many dogs like cat food. I don't know for sure if it "just tastes better", or, they think they are getting something better ! It is more rich than dog food and the dog may get a loose stool so I don't think you want to send the dog home with that possibility of happening. It won't hurt, it may disrupt the digestive system though.