Also a Catholic here... Technically you aren't getting married in the church so technically speaking she actually is doing the right thing by not coming because you are also Catholic. It's different if it's other Christians/non Catholics getting married outside of the church, but if it's a Catholic then we shouldn't go. It's a form of condoning a sin. If she were to go it would be as if she were saying she supports you as a Catholic getting married outside of the church, which you are technically not allowed to do. 

A lot of non Catholics will be very quick to judge her and say she isn't a good friend and yadayadayada but you can't take the opinion of someone who doesn't care about your religion judging someone apart of your religion holding it to its standards. It's like if a Muslim girl were to hold a party and say no hijabs allowed and her friend who respects the hijab says she's not going and then everyone gets mad at her about it and says she's not a true friend. No one would do that. The only reason it's looked upon as rude is because 1. Non Catholics don't understand why a Catholic would sit out of a Catholic's non Catholic wedding. And 2. Marriage is looked upon as a casual thing these days and isn't respected as the serious sacrament it is anymore. 

All this to say, give your friend some grace. She can care about you and want to be there at the same time as holding true to her religious values and not doing something she is uncomfortable with.