Is what my friend is doing — buying petroleum and selling it at a much higher price, while paying sums to officials amidst a crisis in the country — considered haram or halal?

Exploiting people's needs by raising prices is greed and an injustice forbidden by Islamic law. Selling to a person in desperate need is forbidden or invalid, and the bribe paid in this regard is among the major sins. Therefore, one must refrain from this forbidden act.