the sons mum contacted a solicitor already so that she can…

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There are time limits for claiming relationship property. I cite the section so you are clear.

24Time limits for making applications

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  (1)The following time limits apply in relation to applications made under this Act:

  
  

  
  
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    (a)an application made after a marriage or civil union has been dissolved by an order dissolving the marriage or civil union must be made before the expiry of the period of 12 months after the date on which that order takes effect as a final order:

    
    
  
  

  
  

  
  
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    (b)an application made after an order has been made declaring a marriage or civil union to be void ab initio must be made before the expiry of the period of 12 months after the date of the making of the order:

    
    
  
  

  
  

  
  
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    (c)an application made after a de facto relationship has ended must be made no later than 3 years after the de facto relationship ended.

    
    
  
  

  
  
  
  

In very rare circumstances you can reopen a settlement, but I think it would be next to impossible to do so after 17 years. There would have to have been some very substantial miscarriage of justice, and if she was represented by a lawyer at the time, and this was the deal reached with each party having independent advice, then the chances of overturning the settlement would be about the same as winning first division Lotto.