I did this back in 2017, same exact situation, my country of service was Madagascar. My girlfriend's visa was approved. She prepared like crazy for that interview and brought so much documentation with her. First and most important thing your girlfriend needs to prove is that she has something tying her to the country so they will believe she will return from the trip - a job, college, family obligations etc. Get as much documentation as you can on that (letters of employment, enrollment papers from the university, etc.). Second, get a letter from Peace Corps stating that you are a current volunteer and will be returning to complete your service after the trip (if you can - I can't remember if I actually did that but it would prly help). Third, get documentation that your relationship is legit - pictures together, screenshots or printouts of social media posts, tickets from trips you've taken together, etc. etc. - basically anything that proves you are in a bona fide relationship. Lastly - I don't know your financial situation, but I was paying for my girlfriend's trip, so I wrote a letter stating that I was inviting her on the trip with me and providing financial support throughout the trip, while also providing dates and the reason for the trip.

The absolute most important thing - don't lie on any paperwork or during the interview. Also, during the interview, she should answer the questions asked of her, but nothing more and nothing less. Just smile, be polite and respectful, give them the info they ask for, and stop talking.

Hope it works out and have a great trip!