Hi Calamity Jane,
How did she end up owing $20K for someone else's debt? Did she take out a joint loan with her ex or something?
If she was to file for bankruptcy, she would likely be an undischarged bankrupt for a period of at least 3 years, during which time her trustee in bankruptcy would have control over her assets. A trustee has the power to seize and sell some of your assets however the Bankruptcy Act allows you to keep one vehicle, to be used for transport, which must be worth under a certain amount. If it is over a certain amount then the trustee can sell it and give you back about $6K to buy another car. Also, it is the individual who is bankrupt not the couple. Your son will not be affected by her bankruptcy unless they have joint assets, in which case he will not lose his share in those assets, but her may have to buy out the trustee for her share or sell his share or something to that effect.
Here is some info on assets you can retain during bankruptcy:
Bankruptcy and assets - quick guide — Australian Financial Security Authority
Consider the alternatives to bankruptcy before just diving in, as bankruptcy does have serious long term consequences.