I have a UnionPay card which is not supported by Upwork as a billing Method. So I asked for a master card from a friend. But he is already using his master card on his Upwork account.
Now my question is, can I use his master card on my Upwork account?
As long as the owner of the billing method authorized you to use it, you should be able to add it to your account. If any issues arise, Upwork team will notify you directly.