Is it possible to get another entry while staying in Beijing on single entry 50-days L visa?

Hi everybody, my daughter is coming to Beijing on single entry 50-days L with prepaid fixed-date return air-ticket. We would like to make one-week trip to Vietnam during her stay in Beijing. Is it possible for her to apply for additional entry while staying in Beijing and prior to the mentioned trip? If yes, how long does it take? Thanks in advance for your kind answers.