I think her eating the foil was on purpose. Do you really think that someone who throws a shotput through a chokey door, flings a girl by the pigtails, can lift a car and turn it around would have any problem eating candy with the foil still on it? I think not. It was put in there for a laugh.
In the Willy Wonka movie with Johnny Depp... Augustus takes a chomp out of the golden ticket. Just wanting the chocolate. So maybe, Ms. Truchbull like Augustus, just didn't care. Maybe she figured the taste of chocolate would outweigh the taste of the foil...