Yes pigweed is very toxic to pigs and you should never allow pigs to eat any pigweed as it can be fatal for a pig to ingest pigweed.
Always get rid of any pigweed in areas where pigs are hanging out and never let them ingest it.
If the pigs happen to ingest the pigweed then there's not too much you can do but sometimes the pig may be alright if they only ingested a little bit of it.
But usually the pig will die as a result of eating pigweed or at least become really sick as a result and may die later on.
The reason pigweed is toxic to pigs and other livestock is that the pigweed is associated with nitrate accumulation in the livestock including pigs which is what is potentially fatal for the livestock and pigs.