It's his problem now, the way I understand it is the value of his portfolio increased during your annual tax report and he hold it through while the value of his portfolio got down to the point of 100,000$. So even if his cryptocurrency holdings is now below than his tax liabilities he still has to pay it, I am even wondering why he haven't paid his tax liabilities as of last year. Your taxing department won't probably consider the losses you incurred after every tax report you have submitted. You still have the same liability in their eyes.