I was raised in a large Anabaptist church, and just wanted to clarify something for Catholics unfamiliar with Anabaptist beliefs (I admit I don’t know exactly where and how Baptists differ from Anabaptists, so take what’s appropriate…)
Anabaptists do not expressly believe in “re-baptism,” even though their name means “rebaptized.” They believe in a once-and-for-all baptism that entails the adult consent of a Christian believer who has repented and is through baptism forgiven of the sins he or she has committed. All these conditions must be fulfilled. Infant baptism, for them, is not only NOT valid, it is evil. The rationale is that infants cannot be guilty of sin (no, original sin does not come into the equation) and furthermore, that infants have no say in the matter. The crucial assent to faith in Jesus Christ that an Anabaptist baptism demands is therefore missing at an infant baptism. That is why people who have been baptized as infants MUST be baptized as adults if they want to be followers of Christ (for Anabaptists).
In their eyes, your friend is being baptized truly for the first time, her infant baptism signifying nothing more than a showy and misguided ceremony.
Just to clarify: I am a Catholic convert, and do not mean to imply that Anabaptism theology is true. Perhaps, as some have suggested, you should really engage your friend in a dialogue about what baptism it, so that she understands clearly why the Catholic church baptizes infants, and why adult re-baptism is not necessary. If she better understands your beliefs as a Catholic she may be less resentful if you choose not to attend her ceremony.