This is the intended behavior, but I've definitely had it use a non-default card for a second purchase made shortly after the first even with something else in the foreground. I think this is a bug that may have been fixed, because when I select a different card and then open another app, Google Pay now re-selects the default card when it's brought back to the foreground, which it didn't used to do.

OP's wife may have inadvertently swiped over to the other card while moving the phone towards the terminal if she had the app open, though--it's easy to do. There's no need to open the app at all if you want to use the default card--as long as the phone is unlocked, it'll work.