Thanks jvn - my friend ended up speaking with a family lawyer and I thought it was interesting enough to share the results of the conversation.
Essentially on custody issues the HK courts will rule in favor of the best interest of the child, but there is a strong preference toward where the child "habitually resides", and in this case although they are expats they met and married in HK and had a child in HK, so the child habitually resides in HK even though neither of the parents is a HK resident or PRC citizen. She is now in the position of having to make a case for her to have custody based on the fact that she has been raising their son, but also that she cannot stay in HK because she can't work here, etc, therefore she has a better support system back home. The lawyer says courts will favor the mother on custody, but again, there is a strong interest in keeping the child where he has grown up (in a way a silly argument because he's still basically a baby and they haven't lived in HK that long)...which would essentially trap the mother in HK unless she were to give up custody of her child. Apparently there is a Hague Convention parental abduction treaty that HK signed as well, which means you can't just run back home with your child or HK has the right to petition to bring you back? (Not that I would advise something like that!)
I guess the take-away is that if you are feeling trapped in HK and isolated, which I read sometimes a lot of women in HK are when they come here with their husbands, you should try to make a point of visiting your home country frequently to have some evidence that HK is not the home residence. This lawyer is saying honestly it's best to just have an agreement on custody between the parents because the court can rule in some awful ways. But of course, they are going through a messy divorce so that's not going to be easy for them...