Hard to figure out the facts here from OP's choppy description.

Was the customer offered a choice between Euros and HKD at time of CC usage? Large department stores in Paris like BHV have done that for years. While I despise DCC, it is easy enough to just pick Euros, isn't it?

As I said hate DCC. However, the situation the OP seems to describe is much different to a forced DCC with no choice.