A woman who is in the state of Niddah, can she participate in the sedder and all the Pessach celebrations together with her husband at the table?
She can participate in all of the seder, and Pesach celebrations, however she has to keep the regular harchakot. Regarding the seder she shouldn’t pour the water on his hands, or vice versa, rather someone else should do that. They should also make sure to keep the halachot regarding her serving him, and regarding having a heker between them when applicable.
Have a kosher Pesach