Most international airports (in China, like Beijing airport) would have bank
branches (用银行的那个,不要用专门外汇兑换的那个窗口,因为多手续费) around
the exit. You may ask your mom to exchange some there. Otherwise, just ask
any Chinese friends and see whether they keep extra RMB. 2000-3000 RMB
should be good enough, then your mom can go to any bank branch in China. 现
在很多携程机票收中国信用卡预定,找个关系好的中国国内朋友,以防要用信用卡购买
机票。