What is the Islamic legal ruling on trading currencies via the internet, and what should my friend do with the money he earned if it is unlawful?

For currency speculation (Mudarabah), it is a condition to have actual or constructive possession (qabd) at the time of the contract, and for the sale and purchase to be immediately executed. If the speculation is through another party, then the contract between them must be Islamically permissible. Trading in currencies is considered one of the most difficult transactions due to the strictness of its regulations, as it requires immediate possession and the direct crediting of both currencies to the respective accounts. It is a condition in a Mudarabah contract that the share of both the capital provider and the Mudarib (speculator) be a widely known percentage of the profit, and that the Mudarib does not guarantee the capital except in cases of negligence or transgression. If the speculation does not adhere to these conditions, it is فاسدة (voidable) and must be annulled. In such a case, the profit belongs to the capital provider, and the Mudarib is entitled to a fair wage (ajr al-mithl). Profit is not realized until the capital is recovered, and losses are جبر (compensated) from the profit. Based on this, what the person is doing by compensating part of the capital loss with another part of it is not Islamically correct. The transaction must be annulled, the profit belongs to the capital provider, and the other party is entitled to a fair wage.