Symbolism, Imagery, Allegory
Winnie's toad friend is more of a toad friend-of-convenience. The little guy doesn't really seem to care about Winnie, but she confides in him anyway because she has no one else to talk to:
[The toad] gave her not a glance when at last she ran out of pebbles and sat down to tell it her troubles.
"Look here, toad," she said, thrusting her arms through the bars of the fence and plucking at the weeds on the other side. "I don't think I can stand it much longer." (3.2-3)
If he's so detached from our leading lady, why is the toad so important? Well, lucky for us, Winnie does the heavy lifting, explaining herself that she wants what the toad has: freedom.
"I suppose you're right," said Winnie. "Then you'd be just the way I am, now. Why should you have to be cooped up in a cage, too? It'd be better if I could be like you, out in the open and making up my own mind." (3.8)
It's easier to free the toad than it is to free herself, that's for sure. So when she makes the choice of eternal life for the slimy guy, it might just be because she's not ready to make that choice for herself. The toad won't worry about the moral and philosophical dilemmas that immortality can get us scratching our heads over. Immortality might be for toads—but maybe, Winnie is telling us, it's not for humans.