When using the “fast” variant of the tokenizers available in huggingface/transformers 1, whenever you encode some text, you get back a BatchEncoding 3.
This BatchEncoding provides some helpful mappings that we can use in this kind of situation. So, you should be able to:

Find the word associated with any token using token_to_word 9. This method returns the index of the word in the input sequence.
Once you know the word’s index, you can actually retrieve its span with word_to_chars 2. This will let you extract the word from the input sequence.