There are not set rules about who a watcher can and can not help. But I feel that the Watcher is to a Slayer, what a Donor is to a Vampire. They are both entities that carry their own traditions. Traditionally Watchers are the allies to the Slayer, while in a since Donors are the allies to vampires. And this arrangement also is able to once again contribute to this whole equal theorem.
For Vampires you have Slayers as a way to balance them.
For Donors you have Watchers which balance out the allie section for both Vampires and Slayers, and also it allows both groups to reach out to the human population which both hold key items the others need in order to survive and battle in this so called war.
Although it is safe to say that there is no longer a war.