The Cataclysm will bring a time of great sorrow to ALL people, Alliance and Horde alike. That said, the loss of Ashenvale (one of the last uncorrupted lands left to the Kaldorei) will be a very severe blow, and the hostilities between us and the orcs will flare up worse than ever before, I believe.
Many organizations within the Night Elven people are explicitly following the Silverwing example, defending Ashenvale. These will now either see their purpose lost, or find ways to adapt. I hope the Wardens will do the latter. There are still Kaldorei lands to protect.
Naurion has always at least tried to see both sides of a conflict (except with the Naga. Teh Naga is teh bad), and will probably attempt to do so even when Ashevale is overrun, but it will be difficult. At the same time, there will be a lot of shocked, hurt and lost people, both Kaldorei and others, who will be in need of help, and Nature itself will most likely be reeling from the Cataclysm. There will be work to do.