[Nyororin]

Why not ASK the friend for their address?
I`m sure if you told them that you would like to send them a card or gift for their birthday, they will understand. If they refuse to give you their address for that reason - then it is pretty clear they do not want you to have it. If I declined to give someone my address I`d be utterly shocked and VERY upset at them if they researched it somehow and sent me something anyway.