Ricin is extremely toxic. It has an LD50 (the lethal dose for 50% of people exposed) of 30–40mg per kilogram when ingested (its much much more dangerous when introduced to the body in other ways). Guessing Lydia's weight at around 50kg (110 lbs) would lead one to think the LD50 would be no more than 2g of ricin.
It has a military-developed antidote, which has had only limited trials in humans. Ricin is a chemical subject to the Chemical Weapons Convention, in the same schedule as nerve agents Sarin and VX. (All this information gleaned from Wikipedia)
Walt also only told her when she'd already begun to experience symptoms, so less likely to recover even if treated. Even if she sought attention and gave them this information it seems unlikely that she would recover. Even if she's not been given a lethal dose, survivors of ricin often suffer from long term organ damage, so she would have a pretty miserable and presumably shortened life even if she does survive.
I think Walt probably weighed her slim improved chances of survival, against the pleasure of knowing that she was aware who had done this to her.
I considered an answer like @Keen's too — perhaps she would not go to a hospital in fear of the attention, but I think this is less likely, even the risk averse would put fear of death above prison.