khmer@nike

Please complete your timeline 
 
She needs a valid  I-551.  Not a travel document. Those are for stateless and asylees. 
 
Look at her green card. There are two dates:

* “resident since”
* expiration 
 
Compare these two dates:
 
* 2 years apart: she needs to file I-751 along with a reason for why she filed late.  She will get back a letter that extends her green card by two years from the expiry  date.  
 
^ 10 or more years apart: she needs to file I-90. She will get back a letter that extends her green card one year from the expiry date.  
 
If these extension letters extend her green card to a date that is before her planned return to the USA then she needs to  call USCIS, say “info pass” and request an I-551 stamp.  She will not get a stamp until she files I-551 or I-90 as appropriate (unless she has a pending N-400 and was pending 6 or more months before her green card expired).  
 
If she needed to file I-751 and leaves the USA without filing I-751, then there is a good chance she will be stranded and unable to enter the USA. It is not likely the embassy will help her.  

I'm reference to I-131 form for travel document.  it's green U.S Travel document

I'm reference to I-131 form for travel document.  it's green U.S Travel document

Form I-751 can only be filed by paper. Don't file too early: https://www.uscis.gov/sites/default/files/document/forms/i-751instr.pdf "Filing jointly. If you are filing this petition jointly with your spouse, you must file it during the 90-day period immediately before your conditional residence expires."
 
The "Petitioner" for that form is the conditional resident. The US citizen is the "Spouse" listed in Part 8.

Maybe read the guides here on VJ about removing conditions?