Rot-weiss-rot karte application (Vienna)

Dear members

I'm facing a bit of a difficulty in understanding how to help my friend move here.. I would offer him a place at mine until we can find a new apartment together, but for the application he needs " Nachweis des Rechtsanspruches auf eine in Österreich ortsübliche Unterkunft (Mietvertrag oder Untermietvertrag, evtl. Wohnrechtsvereinbarung)." And, I am renting a small place that I am not allowed to sublet... I am not entirely sure how to do this! I wouldn't be sublietting it, I would simply be offering him a place to live until we can find something bigger... Does anyone have any idea how to go about giving him an official document to use for the application??