Examples of plant families that contain species that are safe for human consumption and species that are poisonous to humans?
The most classic example if you want to win this argument would be the family Solanaceae.

Also referred to as the Nightshade family, it includes the deadly nightshade or Atropa belladonna and many other plants not safe to eat.

Other members of the family are tomatoes, peppers, potatoes, and more.

Plant families can be massively diverse, and toxicity doesn't really have much relationship to family. Most of the compounds that are found in plants that are toxic are found in other non-toxic plants as well: dose is crucial.