Hi , Arsenal season ticket holder here living in Sweden. Don’t ask.
Anyhow , you won’t be able to rock up and buy a ticket , not officially . It’s a Saturday 3pm
kick off so not many people will sell them on or not go. I.e Sunday 5pm game in late August will have more available as people are still on summer hols or those that dont live In London might not travel.
It’s a category B game I think. Basically he won’t get one unless you go through a ‘dealer’ or a tout , but touts are rare now as its all on cards. There is an arsenal ticket exchange, try googling that.
Nice that he want to see the PL Champions 2015/16 in action