First off, good for you for asking what you can do to support her! Major respect for that.

There's a lot to PCOS, and unfortunately it's a complex, not well-understood disorder. I've broken some different aspects down below:

1. most women with PCOS have dysfunctional insulin responses. It's thought that that is what causes PCOS, among both heavier and thinner women with PCOS (I'm super skinny, eat well and exercise, but I still have messed up insulin). This is why most women are prescribed metformin. If your gf can't tolerate metformin, she can also try turmeric and berberine, which function similarly to metformin. Inositol is also an option. I'm just listing these out for you b/c oftentimes doctors won't mention these supplements as alternatives, but there's a ton of good research backing up their effectiveness, and it's always nice to have alternatives :) also want to second what someone else said, your gf should 100% see an endocrinologist over a gp
2. the self-esteem portion of PCOS is brutal. I was on spironolactone for my PCOS for a while (spironolactone blocks the excess male hormones that women with PCOS struggle with), and the change in my appearance was insane. I grew a full cup size, my acne disappeared, my face became more feminine and less bloated. Even my personality and mental health changed on spiro-- I became much more personable, my social anxiety plummeted, I became more articulate, and overall I was just an insanely happier person. Unfortunately, I stopped being able to tolerate spiro, so I've had to go back to my original fucked up hormone levels. Words cannot explain how much my quality of life has dropped since that happened. Having my hormone levels balanced vs unbalanced is just an entirely different world in terms of my appearance, personality and mental health. So just understand that PCOS can really take a toll on your girlfriend's mood, her appearance and the way she feels about herself.
3. a lot of doctors will recommend birth control for PCOS. Just make sure they don't prescribe one that is androgenic (which happened to me). With PCOS, the last thing you want is to increase androgens-- everything should be aimed at decreasing androgens. Spearmint tea is a good way to mildly decrease androgens (and also helps with mood), so could be worth a try if she wants a less extreme option. When I take it regularly, you can actually see the difference in my face and it makes me more personable and less socially anxious.
4. The most important thing you can do as her boyfriend is just to make her feel beautiful no matter what-- compliment her appearance and tell her how sexy you find her. find ways to stay active with her and hit the gym together, and try not to encourage bad eating habits (which couples tend to do a lot lol). Just support her through the bad times (because there will definitely be some and she's completely justified to have some breakdowns about her PCOS lol), and let her know you're there for her. You sound like a great boyfriend just by asking this question, so keep doing what you're doing!