It really depends. Some Chinese people especially people from HK speak Mandarin very poorly. I would believe that it is probably better for you to communicate with him/her in English rather than in Mandarin.
There is no need to force anybody to do anything including which language s/he chooses to communicate with you. There are some Chinese people (not necessarily the second generation) who speak very fluent English (unfortunately not including myself). I admire, am proud of them, and understand that sometimes it may be more appropriate (or context accurate) for them to talk to me in English (whether all or just part of it).
Personally, I always talk to my Chinese fellows in Mandarin; however, I choose to write most of my e-mails or web postings in English. The reason is quite simple: I am really really slow in typing Chinese.