@WillyNilly: Grixis857 is right on all counts. The best word, I think, would be “witness” the marriage, which your friend could certainly do as Maid/Matron of Honor, but as a good Catholic she couldn’t fill the role of celebrant/officiant/presider.
It depends, too, on whether you are Catholic/wanting a Catholic wedding, or if you just want your Catholic friend to become qualified to perform a secular service so that you will have a legal wedding (i.e. have her get “ordained” over the internet in the Universal Life Church or something). The short answer is yes, asking her to do that could put her in a troublesome position, theologically speaking. How troublesome depends, in part, on what you do or don’t believe and whether you and your fiance are baptized, but I’m pretty sure that no matter what, it would be problematic for her.
It’s a little complicated, but, the Church believes that when a Christian (any baptized person) enters into marriage, it is a sacrament. For Catholics, we are obligated to marry in the Church, and if we marry outside of it we are committing a sin and can’t take Communion unless and until the marriage is convalidated (i.e. receives a blessing from the Church). So, in a nutshell, if either you or your fiance were baptized (even if you aren’t practicing) and your devout Catholic friend helps you get married in a non-church wedding, that puts her in the position of facilitating a marriage that the Church regards as invalid, so that would make her guilty of committing a pretty serious sin.
If neither of you are Christian and never were baptized, then I’m not 100% sure. I still think that it would be problematic for her because she would be assuming a function that, according to her beliefs, she should not be undertaking. However, one could make the case that by helping you to get married in a “natural” marriage (i.e. a marriage between unbaptized persons, a non-sacramental marriage) she is preventing you from living in sin … so this might still be sinful but perhaps less serious.
I realize that if you’re not Catholic this all probably sounds like silly hair-splitting to you, but think of it this way: if your friend really is devout, then these are important distinctions because it matters to her whether or not she is in good standing with her own faith, right? The best thing for her to do would be to go directly to her priest with the question and all the relevant details and just ask. Be prepared that the answer will probably be “no,” and that might be frustrating to you, but the most loving thing you could do for your friend would be to respect her beliefs even if you disagree with them and not put her in a difficult situation vis a vis the church.