transit via BJ to Laos for US and South African passport holders

As an American, I will be flying Japan to Laos via Beijing Capital Airport but I will be getting my visa on arrival in Laos. Will I still be able to use the 72 hour free transit policy without a visa for Laos already in my passport?

My girlfriend is a South African, what is the best way for her to apply for a transit (G)? Prior to departure? Or is it possible to do it at the airport in Beijing?

Thank you!

Marc