Should I fill out a I-134 form for partially supporting my GF trying to extend a B2 visa?

Besides the I-539 form, she should have proof of onward transportation back to her home country on or before the stated date of departure, an itinerary of what she proposes to do in the 4 months, and proof of adequate support which could include your I-134 affidavit of support with job letter, bank letter, and last year’s tax return. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.