[sourcery]

Looked it up: доверенность can refer to any of "power of attorney," "letter of attorney," "warrant" or "authorization." Given the context, I think a written letter signed by the passport holder authorizing Ruslan (full legal name, and relationship to passport holder) to take possession of the passport should suffice. It's worth a try. If that's not sufficient, they should explain what they require in greater detail.

[MomwithHope]

Thanks to all for helping me. I called the embassy in New York to verify and it can be a simple letter written by her. Now I just have to communicate that to her over Skype this weekend. Her brother needs to bring a copy of her photo id with him. It’s a 7 hour drive to Moscow and a snag we did not expect but I’m sure they will manage. Thanks again!

[cunning_fish]

warrant = ордер, a kind of document issued by a judge, authorization = разрешение, an act by government body to permit something. доверенность = a letter of individual who allows someone to do something affecting rights of an author.