To give more info.
My friends situation is that they have no idea how much money they owe, the IRS does not know about this money because it's simply potential taxes owed during anonymous crypto trades, maybe. As far as the gov is concerned, they owe nothing because they aren't aware that anything even exists.
If the trades associated with any if the centralized exchanges then IRS will know about the earnings made through the cryptocurrency so you have to oay whatever the tax needs to be paid or else you ( friend) will be in trouble because on many occasions US forced many tax evaders who moved to EU to handover them to US because they committed tax evasion but the scenarios or different when people moved from other countries especially from 3rd world countries EU government denied the request of respective governments.
If the person has to move to another country yet then its better to consult the financial advisor who may guide better because they know the laws and probably the loopholes if there is any.