Heya, I have had a similar issue with trying to buy 1500 with of AC Exchange Tickets.
I made a ticket to support about it but not sure how long they will take for a reply.
I am worried that my issue is because my Microsoft Store is a different email/account than the email/account that I selected to play the game on which has my gamertag. I am curious if your friend is in a similar situation where they have selected a different email at the game title screen than the email that is their Microsoft Store account.