Cancelled wedding-do I have to reimburse bridesmaid's airfare?

Morally, yes.  Legally,  if you did not invite him to the wedding, I don't think so.  However, usually bridesmaids are not expected to attend the function without an escort/date for the reception unless the request states singles only.  Are they still able to use the tickets?  What's to stop them from taking a vacation in your area?  Were you expecting the bridesmaid to attend by herself?  Was that made clear to her before the tickets were purchased?  This might be a good case for Judge Judy!

Pay them both. You caused the problem not them. Abd return the gifts.

As stressful as this must be for you, it's probably better to pay her for both tickets - some things aren't worth the fight and ill will.  Of course a married woman, who was asked to be in the wedding party, is going to bring her husband.  I'm sure you knew he would be attending when you asked her to be in your wedding party.

We don't know why you cancelled the wedding, but obviously the relationship wasn't right, so it's better to cancel the wedding, and pay much smaller costs now than later in the form of a divorce.

However, I'd say next time, don't ask her to be a bridesmaid!