I see your point, and that would be a nice thing to implement in the future.
Having said that, the great thing about bitcoin is the ease of transferring funds.
It would literally take seconds to transfer the needed cash from your BTC address in Xapo to hers, so that's not a huge barrier at this moment.
That is of course a solution, but only "practical" if we are at the same store together. Else she has to phone me and ask me to send funds to her account.
What if I'm unable to come to the phone at that moment? She has to stand there, waiting in the shop for me to get back to her? Terrible WAF there...
You've got to see the fact that this is not practical at all, even if I were to be standing next to her, to have to transfer funds.
Additionally, we do have multi-user wallets.
While you can only have one card per account, you can create as many secondary wallets as you want, and they could require permission from all parties in order to withdraw from it.
I fail to see how that would help the aforementioned situation...
Now she would need my permission even if the balance is enough to make said purchase...