I am by no means an expert on the Catholic church, but I am pretty sure that some convents still require a vow of poverty - giving up all personal posessions. In that case, the nicest gift would be an offer to help her do whatever she plans to do with all of her stuff. If she is allowed to take personal belongings, a journal might be a nice gift.
As far as a cake goes, I would probably stick to one with just flowers on it. If you wanted to write something, "Best Wishes" would be appropriate. I don't know that they have cards for this occasion, but a Catholic bookstore might. You could look up a local one on the internet.
As far as writing a personal note on a card, if it was me, I would just wish her well. If you admire her for the step she is taking, let her know. If you wanted an appropriate scripture to quote, Matt 19:21 & 29, Matt 22:37-39, and Mark 1:15-18 are all fitting for the occasion. More than anything, she will probably just really appreciate your friendship and support.
S.