If it's a single, universal code, then it can be used as many times as there are game cards for XY and ORAS. However, from what I can gather about the HA Legendary Birds, their codes are unique and distributed individually via newsletter. In this case, because the codes are not plurally revealed, then logically they are single use only.
My advice is to use the code yourself first, then see if your friends can use it. Or, give it to one of them and see if you can use it. My gut and previous experience with these kinds of codes say that they will be one-use only, though, so proceed with caution.
