What is the common language being used among the locals in places like Beijing, Shanghai, and Xian? I'm assuming it's Mandarin and in HK it's Cantonese? Do the locals (such as the people at the hotels, markets, shops, restaurants, etc.) in Beijing, Shanghai, and Xian speak only Mandarin or do a lot of them speak both? I'm also assuming that most of the staff at the hotels speak English?
I'm sure people who speak Toy-San are probably very hard to find, except of course in Ghuangzhou and the other inner provinces.

You will have no trouble finding English speakers in hotels, restaurants and other tourist areas in the places you are going. Even many taxi drivers speak a little. English menus in restaurants are fairly easy to find, if hilariously translated. The museum in Xian, while very good, does not have a lot of English signage. If you can get a guide there, that might be more helpful. Same with places like the Forbidden City where there is almost no signage, and the Temple of Heaven. Of course a guidebook there or the audiotape at the Forbidden City is fine.

Locals in Beijing speak Mandarin to one another. Locals in Shanghai speak Shanghainese to each other, and most likely Mandarin to other PRC nationals who are in Shanghai, but I have sometimes had PRC business people speaking English to one another as that was their only common language...few people in Hong Kong speak Mandarin, as Cantonese is the local and prevalent dialect, but English is very widely spoken. Not sure of the dialect in Xian, but again for your purposes you will have no trouble finding English speakers.

I think Toy San is another term for Cantonese, so if you speak that, you will have fun in Hong Kong, esp if you are not Chinese, as it blows locals away when foreigners speak good Cantonese.

Thanks Cicerone. My friend was asking me this question. He's going on a tour to Beijing, Shanghai, Xian, and HK. He doesn't speak or understand Mandarin, but he does speak and understand a little Cantonese, and was wondering how helpful that might be. Toy-San is another dialect which is similar to Cantonese. He's fluent in that and was also wondering how useful that might be. I'm assuming probably only in HK.

Cantonese won't be useful in the north. Just stick with English, or hand signs. He'll be fine.

Do you know if Toy-San will be useful in HK??

Very few people in Hong Kong, especially younger ones, will know Toi-shan. I grew up there, and certainly didn't. My parents lived there for 50+ years each, and they could hardly understand any.

Only useful when you have family who were Toi-shan.

My experience in Beijing was that while there may be a reasonable number of English-speakers in and around the high-priced central Beijing hotels, the Wangfujing Dajie shopping mall and other obvious (Western) tourist hangouts, that's about where it stops for all practical purposes. As for cabbies with a little English, in our experience this was limited to "Hello!", "OK!" and "Bye-bye!" Admittedly we stayed at a 3-star hotel that attracted few foreign guests and mostly hailed cabs in the street, first making sure of course that we had our destination written in Chinese characters. Our hotel always had one English-speaking hotel staff member on duty, but like many other Chinese it was evident that she hadn't learn from native English speakers and was nervous about using it. Often it was best to write a question down in English, but even then the word "acrobat" defeated both her and an English-speaking guest who tried to help.

The point is that your friend should be aware that English-speakers are still a very small minority in China, certainly when away from the cosseted atmosphere of western-style 4*/5* hotels. However, with a little planning and some creative sign language this is not really a big problem. Major street and many other signs are in English, and airport and even China Telecom progress announcements are repeated in English. (Don't expect this in bus stations, though.)