Asking about what's next is great for conversation in any case, whether OP's colleague is a friend or not. Maybe they don't have future plans but want to talk about it anyway to feel out the opportunities. It can be helpful and it stays forward-thinking.

If it was a surprise, there's also a non-zero chance they were fired. "Not a good fit" is vague enough to not really mean anything, but good enough to be able to move on with.

Besides that, OP is going to have a very short question if their idea of speaking to their colleague is to interrogate them, and what the OP is thinking about feels very official.