Housewarming parties aren't major gift giving events, but if you want to do something nice that's certainly always a sweet and loving thing to do for a friend.
You could ask her if she needs anything in particular, or you could send a surprise. My first thought is to send flowers (in a vase, or a flowering potted plant) the day before the party. Flowers are usually well received and considered traditional "congratulations" gifts, plants tend to be a traditional housewarming gift, and they would also be a nice way to 'contribute' to the party because they are beautiful and will be part of the festive decor. Alternately sending a festive food gift the day before the party might be nice, as they can put it out as part of the party spread, or save it for themselves as a treat since moving to a new house is often time consuming (not a lot of time to grocery shop) and expensive (not a lot of extra funds to treat ones self to non-house stuff).
Asking her if she needs anything is totally fine if you have that kind of relationship. Some people are very uncomfortable asking for gifts or even answering that type of question though, so it might not garner you much information.