M.,
I think it's wise for your daughter to see the gynecologist. I would be hesitant to take any medication or birth control pills, though.
In my case, I suffered with some weight and acne, plus irregular periods for years. I lost 50 lbs. working out and eating right, and was finally diagnosed with PCOS and endometriosis (birth control pills can exacerabte the PCOS, as can extra weight; per a British doctor, a majority of girls suffering from acne have PCOS but aren't diagnosed by the dermatologists) by a reproductive endocrinologist, then finally gluten intolerance by a nutritionist. In my case changing my diet finally solved my hormonal imbalance and got my cycle normalized so I could actually get pregnant. It also gave me the most pain-free period I had had in my life.
So, in addition to the ob-gyn, I would recommend a nutritionist who could verify any possible underlying food allergies or problems that could be causing the weight gains, infection, and hormonal imbalances. After my diagnosis, I came across a British nutritionist's website, where she indicated that in her patients with painful periods, removing wheat corrected the pain in 80% of the cases.
Another poster commented on the endocrine disrupters in our food supplies. As someone with a condition sensitive to that, I can vouch for it. Also, most folks don't understand that even our grains are different now--wheat has 6x the gluten it used to have 100 years ago. Is it any wonder more folks have a hard time digesting it now?
I would also recommend the nutritionist in lieu of the gastoenterologist from a lifestyle perspective. Gastric bypass may seem like the "only" or "easiest" option to your daughter, but has lifelong effects. I've known several people who have; none had serious complications, but they had to change their lifestyle more dramatically (and permanently) than I did to lose the weight I did. In my opinion, you have to change either way. The nutritionist could help get you on a healthy diet plan, and help keep you there . . . enabling her to live a "normal" life. (The one I've worked with is way better than a dietician. A 45-minute appointment every month that is totally Q&A on whatever I need help with. A great way to have the support you need to make a lifestyle change.) Make sure they are board certified. Either way, her diet will have to change radically. I think her lifestyle and diet would end up being more "normal" (yet healthy) in the long run going with a nutritionist than having surgery. It's also a great time for her to make a change like that while she has your support, and the rest of her life ahead of her.
I don't suggest diet change lightly. I dealt with the diabetic diet while pregnant, and now there's a lot of food I can't eat because it literally makes me sick. I also know diabetics who don't eat the way they should--because they aren't willing to change (or eat things more in moderation)--and it's literally killing them. (One acquaintance had a very lingering few final years, another is set to go the same way, and my mother isn't very far behind.) Our medical profession handles this with a few classes and medication--and usually nothing for the emotional and social implications, so most patients fail to stick to their diets consistently.
Diet change is very hard, very cultural, very emotional. It can also be hard to eat healthy when most folks around you don't. It takes a lot of resolve, effort, knowledge (how to cook and shop), time (to cook), and sometimes more money than eating poorly. The more support she has (you, nutritionist, etc.), the better. But the results--more balanced hormones, better sleep, self-confidence, health, better fertility, longer life, better quality of life--are so worth it. I've kept off those 50 lbs, was able to get pregnant, didn't permanently gain weight after my first child, and this time didn't need blood sugar medication while pregnant.
Another option is to see a regular endocrinologist. However, they will most likely just want to put her on medication instead of solving the problem.