The woman in question lacks tact.
Any sexually experienced woman should know that critiquing a man's sexual performance can be a touchy subject, and she should be fully aware that doing so in a manner in which the man has no true control over is in extremely poor taste and inconsiderate, never mind the fact that her circumcision claim is questionable to begin with.
I am a huge proponent of sexual communication. I highly encourage it with the women I'm sexually active with. But such communication should always be productive, exploratory and experimental in nature. Problems should be addressed with suggestions and solutions, not blame.
It should be a fun process.
@flowtheory, your discussion with this girl should be focused on how this woman communicates such issues. You must let her know you do not approve in the manner in which she approaches it; that you will expect her efforts to reflect the nature of communication I described above. If she cannot do such a thing, there is little reason to carry on with the relationship, as there would be no fun in it.
Regardless of who is to blame here, the communication sucks and must be corrected.