tofu

there is no such thing, family debt.  when a couple is separating, they only spite the assets, not debt.  be careful.

if you need legal help, you can either call the help line of the legal society, which they can give you simple advise by phone or refer you to a lawyer.  or you can make an appointment to see an ubc student lawyer.  can't member the name of the org, but you should able to search online.   

if he is collecting welfare, there is not much you can do.  if he moves to another country, your mum can trace him and ask him to pay for the predetermined child support pmnt.   the best case i knew is the guy moved back to europe and his wife can has the support pmnt directly deducted from his payroll.  so he doesn't has a chance to touch the money before it is sent to her.  not sure does it work in hk

if they have a house, make sure he cannot get a 2nd mortgage on it.  some of the private companies approved 2nd mortgage with one signature even though both husband and wife are on title.  of course much higher interest rate and smaller amount.