(Dota 2) Where Can I Find My Beta Invite Keys?

have you tried clicking games in the top menu in the main steam window, then manage gifts and guest passes?

If he has any they should be in his Steam inventory. He can check his inventory from his Community profile

If he got it by activating a key, then it probably did not come with 2 extra invites which new keys don't provide. Otherwise it's in Community > Inventory > Steam tab.

They should be in the inventory..

Thanks for the help everyone, it seems like he didn't get the guest passes.
He's gonna try to blag it to get his hands on some, lol. :D