You're friend can use the Park 11s tool or the Pedros 11s tool. They come without the somewhat steep price of the Campy tool.
Also, there are people joining 11s chains with 10s chain tools, and just not riveting the pin. Whether that's acceptable or not depends on what level of risk you assume exists and are willing to take. Lastly, if you do a search--googlify, that is--some folks have made their own chain riveting tools themselves.