~ Food Has Power ~
Yes, endometriosis can cause a lot of pain and vomiting, so it may be that. Why are you thinking PID? Does she always have pain or just during menses? Fever? Often times women with endometriosis get treated for PID when they show up in the ER because of severe menstrual cramps. The antibiotics aren't going to help if it's endometriosis and the pain for most women with endometriosis resolves every month.
The problem with endometriosis is it's hard to find a good pelvic pain specialist to do an evaluation. Any GYN will claim to do one, but it often ends in misdiagnosis or a long road of inadequate surgeries and inappropriate treatments. Are you in the US? The Endometriosis Association has local chapters which will often know which docs are the best for evaluation and treatment if she wants.
Magnesium is the single most thing that helps me! Good nutrition overall will help, but extra magnesium for sure. Calcium and fish oil are helpful too.