Do I need fill the I-90 form for her now or after she comes back?
-Normally no and she can file it after she comes back to US. But not hurt to fill it and bring it, anyway (it is simple).
How long would it take to get the transportation letter?
-Immediately. Read the following website (click to open APPLICATION FOR TRANSPORTATION LETTER with required AFFIDAVIT and FORM I-193) for more information on the Transportation Letter application and it is possible she can apply for the Transportation Letter in Shanghai (she may contact the US Consulate in Shanghai after she submit the application to Guangzhou or Beijing):
http://shanghai.usembassy-china.org.cn/green_cards_holders.html
Thank you!
Merry Christmas and Happy New Year!