The issue was resolved by returning the original 2 BTC deposit. That's OK. But there is one more issue. The player can obviously not be allowed to withdraw 5 BTC. He can also not be allowed to withdraw 3 BTC (2 BTC original deposit + 1 BTC of earnings) because that 1 BTC of extra profit came thanks to wagers made with the bonus funds as well. But if he played with his own 2 BTC as well, shouldn't he be allowed to withdraw winnings that would represent the profit he would have made if he placed wagers only with those 2 BTC? In this case, instead of 1 BTC of profit, only 0.5 BTC. Maybe that would have been the most optimal solution.
I don't think returning 2.5 BTC would have been better. I assume that's not according to the deposit conditions. The whole point of a deposit bonus is to make you wager more (and increase the chance to lose). It's no charity, it's designed to give the casino an edge, not the player. If the conditions aren't met, the user can't withdraw winnings. What if the user would have lost instead of won? He wouldn't be allowed to keep his 2 BTC that way either.