Is what happened forbidden in Islamic law, and is it obligatory to return the amount in Saudi riyals and demand the friend to pay the amount in euros, knowing that I will not benefit from anything, but my friend will lose?

For credit card transactions to be permissible, they must not involve a usurious contract. There is no objection to using another person's card if it is religiously permissible.

If you paid for your friend's ticket with your card, this is a loan from you to him in euros. You have the right to demand repayment in euros or accept repayment in another currency at the exchange rate of the day of repayment.

If the amount your friend paid was less than the exchange rate on the day of repayment, there is no objection for you not to refund the difference, because a creditor can waive some of his right. This is unlike a debtor taking more than what is due to him, as the Prophet (peace be upon him) forbade profiting from what one has not guaranteed.