Bugpuss: I agree with the other posters that coating ehr arrows with the snakes venom should cost her extra actions - dependng on how harsh you want to be, it could either be a ME to coat , or a full-round action to caot and shoot a single arrow = it's all just semantics at that point.
For easy bookeeping, keep the Effect and the Dc the same.
If your players have Know (Nature), Handle Animal, or maybe Alchemy? you should at least give them a shot at harvesting some poison. But don't forget about the 5% chance of poisoning yourself when handling poison!