I have visted twice - once for 2 weeks and once for a month. My brother in law has worked in Asia for the last 20 years, including many stints in China. Feel free to pm me if you have any specific questions. The degree of difference willl really be dependent upon where he works. Shanghai for instance is so Westernised you can get along very well with English as your only language and the cultural shift will be minimal. Beijing is also very westernised and you can get along quite well there with very little difficulty. Once you start to get away from these centres, things change. In some places a western face is almost as rare as a movie star. I am 6'1" with blond hair and blue eyes - in some places I had lineups of people asking to have their photo taken with me, because it is so rare to see a western culture. My wife, who also had blond hair got even more requests
Even with the recent economic growth, if you start getting into some of the regional areas, the westerniation is much less , and the smaller the area the bigger the cultural differences. The thing that I have always found amazing is the potable water situation. Last time I was there 90% of the water - even in the major cities- was not potable, even in major hotels such as Holiday Inn, don't drink the water. This has some pretty significant ramifications - it means soups at restaurants (found this one out personally - Montezuma's revenge Chinese style ), soft fleshy fruits and many other things we take for granted are off the list of things you can use / eat / drink.
But it is an amazing experience, and I would take it at a heartbeat if offered;really far too much to expereince and touch on here - much of it fascinating from the Great Wall to the Summer Palace to the Terra Cotta Warriors to some of their Acrobatic shows and theatre.
I'm sure they will have the experience of a life time they enjoy!