It's great of your friends to take in this little one, there needs to be more people like this. People need to stop putting human emotions on the faces of animals. They don't have the muscles to express emotion like we do. The only proper way to tell what an animal is feeling is by reading the body language. Ears forward - alert. Whiskers forward - curious, interested. Her ears aren't back, which might be fear or preparing to pounce/attack. She seems more interested in what's going on around her then fear of the people around her. I don't care for people saying "Animals should be FREE, in the wild! Zoos are evil!" In the wild, the animals could struggle for food (competing with humans), be endangered by overwhelming predators (and humans are predators of every animal), and are catching diseases, and of course, there is the problem of the wild being destroyed by humans. If the zoo is good, makes sure animals aren't ill, give the nutrients they need in the for they need and it's in a natural way so they can use their instincts (ex. big cats "hunting" for the raw meat, not just having it handed or to them) and making sure the animals mate so their species is not endangered, then I don't see the problem with animals living healthy, long lives in captivity. As for adapted animals (like foxes) there isn't a problem if the people are competent, there are ways to keep the natural instincts in them (like making them "hunt" for their food) and it helps them to not be scared of humans if they are interacted with in a respectful manor.
Well, some emotion can be expressed facially, (domesticated animals can slightly adapt to human social standards based on reactions from their owners [and vice-versa]. Hence why "Puppy dog eyes" is even a term.), but yeah, it is mostly body language due to muscle structure from social evolution.
But I do agree with what you said. "Freedom" doesn't mean "Happiness".
Even us as humans know that. If you're in America (or any other country, really) you'd know the really meaning of freedom not equaling happiness. We all do. ...unless you're very rich then... you probably do have both... D: