I wonder if she’s aware she’ll likely have to divorce her husband. The Qu’ran pretty clearly states that a Muslim woman is now allowed to be married to a non-Muslim. Well, it’s certainly none of my business.
As for your friend’s decision, I don’t think it is as cut and dry as others have suggested. It isn’t as if she’ll cease her conversion if you don’t go. What you may do is effectively cut out one of the good, Catholic influences in her life. If our Holy Father can visit a Mosque and pray with Muslim’s then I don’t see why you can’t attend her ceremony. Be sure to express your concerns with her actions, ask her to take a second look at the Catholic Church, and offer to answer any objections (your fellow CAFers will no doubt be happy to assist you in this) to the Catholic faith. In this way you remain friends, continue to be a good influence, and let her know your objection to her action. Outright refusing to go may be a stronger condemnation, but I fear it does more harm than good.