What is the ruling on someone who entered into forex trading, stipulating to the funders that they would receive 30% of the capital monthly in exchange for bearing half the loss, then lost everything? Is the money unlawful (haram), is he compelled to repay the amount, and how does he repent to Allah?

The transaction mentioned is a void Mudarabah (partnership) because you took money to invest it with a guaranteed fixed percentage of profit or half of their funds in case of loss. If the Mudarabah is void, then all profit belongs to the capital owner, and all loss is borne by them, while the Mudarib (investor) is entitled to a wage similar to their effort. Therefore, you are not responsible for guaranteeing the loss as long as it was not due to negligence or transgression on your part. You must seek forgiveness for these transactions. It should also be noted that Mudarabah through the Forex system involves Sharia prohibitions.