The "reason" given was the truth which was for her to come visit me for my birthday, and meet friends and family. This was carefully written in both her letter of explanation and my letter of invitation. She has a kid over there, so we included a small mention of her wanting to come back to visit with her next time. She included the kid's birth certificate and affidavit for solo parent ID. I looked up examples of other people's letters of invitation and created mine using those as a general guide.
The fact that I had visited there twice already recently and we had traveled together outside of the Philippines was clearly explained and the proof of our relationship was included (just a few pictures of me with her and her family).
I made every effort to ensure her application was as perfect as possible in their eyes. No spelling or grammar mistakes, no poorly formatted parts, also filled out one of the "optional" documents, provided as much proof of everything as I could even if it wasn't explicitly called for. Even though she doesn't have savings, I still made her include months of bank statements, and I included mine as well. Tax documents for both of us.
In the end I was able to fill out all the required sections but I was still sure it would be rejected considering she's not employed, has no savings, and fairly little to prove she would return. I'm shocked that it was approved and kinda freaking out now hehe. Can't wait! I wonder what kind of visa they will give, since they started "automatically considering everyone for multiple entry", it's no longer a separate thing to apply for.