Did he eat the seeds raw? This paper suggests it may be possible to reduce the toxin through simple means:

http://onlinelibrary.wiley.com/doi/10.1002/%28SICI%291097-00...

  Ordinary cooking and pressure cooking of pre-soaked seeds 
  were found to be most effective in reducing the levels of 
  all the natural toxicants examined...

  These findings and the high water solubility suggest that 
  a simple and effective means of detoxifying Lathyrus by 
  removing this neurotoxic amino acid may be practicable.