You know it's possible to use ice to start a fire.
Here's a story of water starting a fire.
UPDATE 1: This space reserved for the first commenter with video showing that they used a bowl of water as a lens to start a fire. Get out there, Mythbusters! I will link to your video clip!
UPDATE 2: Our attempts with a large, hemispherical mixing bowl and black posterboard lead me to doubt the likelihood of this explanation. The dog bowl in the story is a frustrum of a right circular cone, so lensing effects don't seem plausible. One might retreat to a position that the rim of the bowl acted as a mirror. But reflections from a water-glass boundary? The intensity simply isn't there.
In my Mythbusting opinion, the "dog bowl as magnifying glass" explanation holds no water.