There are two possibilities here. If the debtor did not schedule the $50,000 as an asset in the bankruptcy, then the failure to schedule the asset may be bankruptcy fraud, regardless of anything else. Were that true, and the trustee
discovers the error, then the trustee could ask the court to reopen the case, and the court would likely to do so, so that the asset could be recovered. This assumes, however, that the debt is worth recovering. The trustee has the authority to decide not to try to recover the value of the asset, if he/she believes it will not be worth the effort.
The statutory authority to reopen the case is found in Bankr. Code
Even if the trustee were to decide to request that the case be reopened, a bankruptcy court
may decline to reopen a case to recover an asset that the trustee made a deliberate informed decision not to administer. In re Adair (9th Cir. BAP 2000) 253 BR 85, 91 (court refused to reopen Chapter 7 case in order to revoke trustee's technical abandonment of lawsuit that debtors had properly scheduled as having value in “unknown amount” (although case ultimately settled for about $430,000)).
The botXXXXX XXXXXne is that if the debtor didn't report the asset, then the trustee will try to determine if it's worth the money to go after the third-party debtor (ex-spouse) to recover the debt.
The last thing that would be considered would be a bankruptcy fraud investigation, because that doesn't make the trustee or the unsecured creditors of the bankruptcy debtor any money -- and it costs the federal government money to investigate and prosecute. Only if there were some sort of conspiracy to hide the asset, between the two ex-spouses, is it likely that a criminal investigation would occur.
Hope this helps and happy new year.