Chinese is a little tricky. There are a lot of Chinese nationals in the US now, but creating guide materials for them can be kind of complicated because there are so many regional dialect variations in the way that things are written. (I work in a place that has a lot of official visitors from China, so we deal with this often.)
I think that a lot of the attractions at WDW can be appreciated even if you don't speak English at all. The trickiest part of it for them is probably going to be just getting there and getting situated at their hotel. I think it would probably help if your friend looked into Chinese tour operators that do trips to WDW, and have her parents possibly stay where they stay, or even take one of their tours if possible. Onsite has advantages, too, because they would be able to take ME and not have to deal with local transportation.
It also might be nice if she made them some Disney-specific phrase cards, with the phrase written on one side in Chinese, and in English on the other, so that they can show the cards to CM's if they need assistance in the parks or at their resorts. That way, though there are no Chinese park maps, at least they can have a CM mark the spot on the English park map so that they can find it. (Their daughter could also make them some custom maps from home and translate the information for them.)