She feels really lost right now and so do I
Couple things: there's what you can do, and then there's what she can do.
I love the examples given of making friends with other significant others. Cause then they are meeting people who get to deal with the lunacy of being the vet med spouse. Even after graduation, being the vet med spouse is hard.
Moreover, you can't make friends if you don't expose yourself to others. You guys can either together or she herself separately can take the plunge to go do something new. Local book club, running club, club sports team, crochet group, quilting group, painting lessons, volunteering. Etc.
Prioritizing your relationship would be the most important step. Regular date nights, meals/activities together, etc.
Something left field: long distance. Both of my major relationships (5years during undergrad and the year following, and then my only relationship during vet school and my now husband) were long distance (5hr drive for the first, 16hr drive for the second). The first didn't work out, in part due to vet school. He didn't want to wait for me to get in and get through; he also didn't want to live wherever I would go. The second worked great because he understood the long term outlook much better. He stayed in Denver with his support system while I was in school. He came to visit 1-2 times per year. We talked every day. Etc.
Ultimately, relationships are both a team and individual sport. Unfortunately, when a significant other is in a doctorate program, the other spouse gets set on the back burner, whether for the better or worse. She needs to both prioritize what she can do to make the situation better by getting out there and making friends/her own support system, while also realizing this won't last forever. Meanwhile, you have to prioritize your relationship with her over other social stuff, while also making sure you get through the program.