It is proven that babies feel pain, but I doubt that you can convince her of that if she wants to believe something so obviously in fantasy land. She will believe anything to justify her desire to cut genitals.
Pain is not the issue, because even if there was no pain in circumcision- it would still be wrong- and if you base your argument on the pain- then anesthesia would make it all OK, and you know- and I know that the violation is not simply because it hurts.
Also we all know that circumcision is not necissary- no one maintains that it is necissary- this is not a good platform to stand on either because even if you can convince her of it not being necissary, which is OBVIOUS... you still have not proven to her that it is wrong.
I suggest that she should be taught about the sexual function and value of the prepuce, and when she understands that- then the ethical implication of genital cutting will be clear to her. I doubt that she will have any interest in that though.
Here are some sources about pain- just FYI- good luck:http://www.cirp.org/library/pain/taddio2/
"Preliminary studies suggested that pain experienced by infants in the neonatal period may have long-lasting effects on future infant behaviour."
" It is specious to hold that an infant's sensibilities are not sufficiently developed to perceive pain. "http://www.ama-assn.org/ama/pub/article/2036-2511.html
"There is considerable evidence that newborns who are circumcised without analgesia experience pain and physiologic stress. Neonatal physiologic responses to circumcision pain include changes in heart rate, blood pressure, oxygen saturation, and cortisol levels.36-39"http://www.aap.org/policy/re9850.html